C#: Add some tests for cs/useless-if-statement.

Author: michaelnebel

csharp/ql/test/query-tests/Useless Code/FutileConditional/FutileConditional.cs

@@ -0,0 +1,29 @@
+using System;
+
+class FutileConditionalTest
+{
+
+    public void M(string s)
+    {
+        if (s.Length > 0) ; // $ Alert
+
+        if (s.Length > 1)
+        {
+        } // $ Alert
+
+        if (s.Length > 2) // GOOD: because of else-branch
+        {
+        }
+        else
+        {
+            Console.WriteLine("hello");
+        }
+
+        if (s.Length > 3)
+        {
+        }
+        else
+        {
+        } // $ Alert
+    }
+}

csharp/ql/test/query-tests/Useless Code/FutileConditional/FutileConditional.expected

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+| FutileConditional.cs:8:9:8:27 | if (...) ... | If-statement with an empty then-branch and no else-branch. |
+| FutileConditional.cs:10:9:12:9 | if (...) ... | If-statement with an empty then-branch and no else-branch. |
+| FutileConditional.cs:22:9:27:9 | if (...) ... | If-statement with an empty then-branch and no else-branch. |

csharp/ql/test/query-tests/Useless Code/FutileConditional/FutileConditional.qlref

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+query: Useless code/FutileConditional.ql
+postprocess: utils/test/InlineExpectationsTestQuery.ql

csharp/ql/test/query-tests/Useless Code/FutileConditional/options

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+semmle-extractor-options: /nostdlib /noconfig
+semmle-extractor-options: --load-sources-from-project:${testdir}/../../../resources/stubs/_frameworks/Microsoft.NETCore.App/Microsoft.NETCore.App.csproj