Fix build warnings in Test project (#178)

The build warnings were of these type (mostly reported by xUnit's Code Analyzers):
- Fixed wrong parameter order in xUnit assertions (can lead to poor error reporting in test failures)
- Unused code was removed
- Correct assertions were used (e.g. Assert.True/.Contains/.EndsWith)
- Public non-test methods on test classes were made private

src/Test/L0/Worker/VariablesL0.cs

@@ -150,7 +150,7 @@ namespace GitHub.Runner.Common.Tests.Worker
                 string actual = variables.Get("no such");
 
                 // Assert.
-                Assert.Equal(null, actual);
+                Assert.Null(actual);
             }
         }