`CSharpCompilationUtil`: make it `IDisposable`

[ForNeVeR]

In tests, we have a class CSharpCompilationUtil that is used to compile C# code.

Currently, it is a static class with a bunch of functions. The tests are expected to invoke these functions and clean up after their termination (i.e. remove the compiled assemblies and whatnot).

I don't think the tests do a good job on that, and most likely they leave a lot of garbage behind.

We should make CSharpCompilationUtil a normal test fixture, inject it into tests, support IDisposable interface and make it clean up after itself automatically.

(Or perhaps just make it IDisposable and make the tests to manage it, without the test fixture part.)

Also, remove TODO[#492] from the code.