Add explicit tests around CompareAssemblyIdentity implementation

[rainersigwald]

Before the .NET Core port, ResolveAssemblyReferences compared assembly identities using a native method call:

https://github.com/Microsoft/msbuild/blob/a939f1547a3f8b91512d0d2d270a8d0f0111e7e7/src/XMakeTasks/NativeMethods.cs#L976-L985

Now, there's a managed implementation that optionally calls that one on Windows:

msbuild/src/Tasks/NativeMethods.cs

Lines 1114 to 1136 in a8efaf1

	// TODO: Verify correctness of this implementation and
	// extend to more cases.
	internal static void CompareAssemblyIdentity(
	string assemblyIdentity1,
	bool fUnified1,
	string assemblyIdentity2,
	bool fUnified2,
	out bool pfEquivalent,
	out AssemblyComparisonResult pResult)
	{
	#if FEATURE_FUSION_COMPAREASSEMBLYIDENTITY
	if (NativeMethodsShared.IsWindows)
	{
	CompareAssemblyIdentityWindows(
	assemblyIdentity1,
	fUnified1,
	assemblyIdentity2,
	fUnified2,
	out pfEquivalent,
	out pResult);
	}
	#endif

We should have tests that validate that implementation using the native method as a reference. This is a nice unit-testing problem since it's functional and idempotent, so we can probably hit 100% code coverage.

Related: #3930, #5973