simple-cipher: add test for random keys

exercises/practice/simple-cipher/.meta/Generator.tpl

@@ -16,8 +16,6 @@ end}}
     end
 end}}
 
-using Xunit;
-
 public class {{ testClass }}
 {
     {{- for test in tests }}
@@ -31,6 +29,13 @@ public class {{ testClass }}
         Assert.Equal({{- test.expected | string.literal }}, sut.Decode({{ test | decode_arg }}));
         {{- else if test.property == "key" }}
         Assert.Matches({{- test.expected.match | string.literal }}, sut.Key);
+        }
+
+        [Fact(Skip = "Remove this Skip property to run this test")]
+        public void Random_key_cipher_key_is_random()
+        {
+            var keys = Enumerable.Range(0, 1000).Select(_ => new SimpleCipher().Key).ToArray();
+            Assert.InRange(keys.Distinct().Count(), keys.Length - 100, keys.Length);        
         {{ end -}}
     }
     {{ end -}}

exercises/practice/simple-cipher/SimpleCipherTests.cs

@@ -28,6 +28,13 @@ public void Random_key_cipher_key_is_made_only_of_lowercase_letters()
         Assert.Matches("^[a-z]+$", sut.Key);
     }
 
+    [Fact(Skip = "Remove this Skip property to run this test")]
+    public void Random_key_cipher_key_is_random()
+    {
+        var keys = Enumerable.Range(0, 1000).Select(_ => new SimpleCipher().Key).ToArray();
+        Assert.InRange(keys.Distinct().Count(), keys.Length - 100, keys.Length);
+    }
+
     [Fact(Skip = "Remove this Skip property to run this test")]
     public void Substitution_cipher_can_encode()
     {