We must set `ResolveAssemblyConflicts=true` before loading
Microsoft.Common.targets (which is loaded by Microsoft.CSharp.targets),
because otherwise we won'd do any conflict resolution at all, since the
variable isn't 'true' when it needs to be.
Also add test.
Fixes https://github.com/xamarin/xamarin-macios/issues/11691.
Rolf Bjarne Kvinge