Fix several warnings in System.Linq.Expressions.Tests build

These warnings were being generated during the build:
```
SequenceTests\SequenceTests.cs(718,38): warning CS0168: The variable 'e' is declared but never used [corefx\src\System.Linq.Expressions\tests\System.Linq.Expressions.Tests.csproj]
SequenceTests\SequenceTests.cs(737,38): warning CS0168: The variable 'e' is declared but never used [corefx\src\System.Linq.Expressions\tests\System.Linq.Expressions.Tests.csproj]
Cast\IsTests.cs(2132,28): warning CS0183: The given expression is always of the provided ('Enum') type [corefx\src\System.Linq.Expressions\tests\System.Linq.Expressions.Tests.csproj]
Cast\IsTests.cs(2177,28): warning CS0183: The given expression is always of the provided ('object') type [corefx\src\System.Linq.Expressions\tests\System.Linq.Expressions.Tests.csproj]
Cast\IsTests.cs(3527,28): warning CS0183: The given expression is always of the provided ('object') type [corefx\src\System.Linq.Expressions\tests\System.Linq.Expressions.Tests.csproj]
Cast\IsTests.cs(3572,28): warning CS0183: The given expression is always of the provided ('ValueType') type [corefx\src\System.Linq.Expressions\tests\System.Linq.Expressions.Tests.csproj]
Cast\IsTests.cs(4427,28): warning CS0183: The given expression is always of the provided ('IEquatable<S>') type [corefx\src\System.Linq.Expressions\tests\System.Linq.Expressions.Tests.csproj]
Cast\IsTests.cs(4472,28): warning CS0183: The given expression is always of the provided ('object') type [corefx\src\System.Linq.Expressions\tests\System.Linq.Expressions.Tests.csproj]
Cast\IsTests.cs(4517,28): warning CS0183: The given expression is always of the provided ('ValueType') type [corefx\src\System.Linq.Expressions\tests\System.Linq.Expressions.Tests.csproj]
Cast\IsTests.cs(5012,28): warning CS0183: The given expression is always of the provided ('object') type [corefx\src\System.Linq.Expressions\tests\System.Linq.Expressions.Tests.csproj]
Cast\IsTests.cs(5057,28): warning CS0183: The given expression is always of the provided ('ValueType') type [corefx\src\System.Linq.Expressions\tests\System.Linq.Expressions.Tests.csproj]
Cast\IsNullableTests.cs(488,28): warning CS0183: The given expression is always of the provided ('object') type [corefx\src\System.Linq.Expressions\tests\System.Linq.Expressions.Tests.csproj]
Cast\IsNullableTests.cs(533,28): warning CS0183: The given expression is always of the provided ('ValueType') type [corefx\src\System.Linq.Expressions\tests\System.Linq.Expressions.Tests.csproj]
SequenceTests\SequenceTests.cs(2001,38): warning CS0168: The variable 'e' is declared but never used [corefx\src\System.Linq.Expressions\tests\System.Linq.Expressions.Tests.csproj]
```

Author: stephentoub

src/System.Linq.Expressions/tests/Cast/IsNullableTests.cs

@@ -467,42 +467,7 @@ private static void VerifyGenericWithStructRestrictionIsObject<Ts>(Ts value) whe
                     Expression.TypeIs(Expression.Constant(value, typeof(Ts)), typeof(object)),
                     Enumerable.Empty<ParameterExpression>());
             Func<bool> f = e.Compile();
-
-            // compute the value with the expression tree
-            bool etResult = default(bool);
-            Exception etException = null;
-            try
-            {
-                etResult = f();
-            }
-            catch (Exception ex)
-            {
-                etException = ex;
-            }
-
-            // compute the value with regular IL
-            bool csResult = default(bool);
-            Exception csException = null;
-            try
-            {
-                csResult = value is object;
-            }
-            catch (Exception ex)
-            {
-                csException = ex;
-            }
-
-            // either both should have failed the same way or they should both produce the same result
-            if (etException != null || csException != null)
-            {
-                Assert.NotNull(etException);
-                Assert.NotNull(csException);
-                Assert.Equal(csException.GetType(), etException.GetType());
-            }
-            else
-            {
-                Assert.Equal(csResult, etResult);
-            }
+            Assert.True(f());
         }
 
         private static void VerifyGenericWithStructRestrictionIsValueType<Ts>(Ts value) where Ts : struct
@@ -512,42 +477,7 @@ private static void VerifyGenericWithStructRestrictionIsValueType<Ts>(Ts value)
                     Expression.TypeIs(Expression.Constant(value, typeof(Ts)), typeof(ValueType)),
                     Enumerable.Empty<ParameterExpression>());
             Func<bool> f = e.Compile();
-
-            // compute the value with the expression tree
-            bool etResult = default(bool);
-            Exception etException = null;
-            try
-            {
-                etResult = f();
-            }
-            catch (Exception ex)
-            {
-                etException = ex;
-            }
-
-            // compute the value with regular IL
-            bool csResult = default(bool);
-            Exception csException = null;
-            try
-            {
-                csResult = value is ValueType;
-            }
-            catch (Exception ex)
-            {
-                csException = ex;
-            }
-
-            // either both should have failed the same way or they should both produce the same result
-            if (etException != null || csException != null)
-            {
-                Assert.NotNull(etException);
-                Assert.NotNull(csException);
-                Assert.Equal(csException.GetType(), etException.GetType());
-            }
-            else
-            {
-                Assert.Equal(csResult, etResult);
-            }
+            Assert.True(f());
         }
 
         #endregion