[msbuild] Add msbuild test for #3791: Skip reference assemblies passed to mtouch

[VincentDondain]

See https://bugzilla.xamarin.com/show_bug.cgi?id=53164.

Our msbuild tests are in fact still using xbuild. Because the test case in #3791 is using NugetPackages added via "PackageReference" it needs msbuild. xbuild cannot "restore" nor is using the right ResolveAssemblyReferences target (that feeds csc with the right assemblies).

Adding the test itself once our tests are using msbuild should be trivial. It's just a matter of building the project with msbuild which will resolve the packages correctly.

[VincentDondain]

There should also be a mmptest to cover the Xamarin.Mac side of the house.

#3991

[VincentDondain]

xbuild fix now being tracked here: #4110