Add Marshal API test coverage for generics marshalling

[MichalStrehovsky]

#103 added support for marshalling generics, but we don't seem to have test coverage for the Marshal APIs with this:

• Marshal.OffsetOf
• Marshal.SizeOf
• Marshal.PtrToStructure
• Marshal.StructureToPtr
• Others?

E.g. Marshal.OffsetOf already worked on desktop CLR because the runtime didn't do a good job of blocking it (and it also raises a question as to whether the code paths this is taking are quite different that made it so that blocking didn't take place). We don't have tests that the returned numbers are actually good.

We might as well throw in a test for marshalling as UnmanagedType.AsAny because I'm sure WinForms is keen to use that (/snark).

I'm making this a .NET 5 issue since this is a new feature.

[elinor-fung]

Moving to 6.0 as test debt we need to address.

[jkoritzinsky]

I've validated that we already have coverage for SizeOf, PtrToStructure and StructureToPtr. We never enabled generic support for these APIs.

As mentioned in the original issue, we don't have tests for OffsetOf and it seems to allow generic types. I'll add some tests for OffsetOf since I don't think we can break back-compat here (doesn't meet the bar).

[elinor-fung]

I've validated that we already have coverage for SizeOf, PtrToStructure and StructureToPtr. We never enabled generic support for these APIs.

Does this include versions of SizeOf and PtrToStructure that take the actual object? I think they also work with blittable generics (in Framework too).

runtime/src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/Marshal.cs

Lines 107 to 125 in ae64899

	public static int SizeOf(object structure)
	{
	if (structure is null)
	{
	throw new ArgumentNullException(nameof(structure));
	}
	return SizeOfHelper(structure.GetType(), throwIfNotMarshalable: true);
	}
	public static int SizeOf<T>(T structure)
	{
	if (structure is null)
	{
	throw new ArgumentNullException(nameof(structure));
	}
	return SizeOfHelper(structure.GetType(), throwIfNotMarshalable: true);
	}

runtime/src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/Marshal.cs

Lines 605 to 613 in ae64899

	public static void PtrToStructure(IntPtr ptr, object structure)
	{
	PtrToStructureHelper(ptr, structure, allowValueClasses: false);
	}
	public static void PtrToStructure<T>(IntPtr ptr, [DisallowNull] T structure)
	{
	PtrToStructureHelper(ptr, structure, allowValueClasses: false);
	}

[jkoritzinsky]

It looks like we also don't block those. I'll add some testing for those cases 😢