Add decompiler accuracy tests as C#/VB.NET/F# snippets

[H-A-M-G-E-R]

Compile the snippets, then decompile them, and check if the code is functionally the same. (The decompiled C# code and recompiled IL code may be different, and the decompiler can only decompile IL to C#, not VB.NET nor F#.)

Use many different versions of the compiler, optimizations on or off.

This is so an entire XNA/FNA game can be recompiled while behaving identically.

[siegfriedpammer]

We already have a correctness test suite that checks behavior of IL and C# programs by executing them. See https://github.com/icsharpcode/ILSpy/blob/master/ICSharpCode.Decompiler.Tests/CLAUDE.md for an overview.

What exactly do you have in mind? Should we add more test cases or languages? We already support legacy csc, all major versions of Roslyn, two versions of mcs; with optimizations on or off, as you can read in the aforementioned document.

It would be more helpful if you could share concrete examples of bugs, so we can adapt the correctness test suite in the area where you found a bug. Thanks!

Note: a full roundtrip of a XNA game is not likely to happen as a test, since it needs a lot of infrastructure work.

[H-A-M-G-E-R]

Perhaps try to decompile hand-written IL code as a sanity check?