…sserts.

Asserts not provided by Xunit are provided by an AssertExtensions class.

Tests that reference System.Private.Corelib directly will use a polyfill implementation based off the old CoreCLRTestLibrary asserts.

All assert methods provided by CoreCLRTestLibrary have been changed to follow Xunit conventions.

Co-authored-by: Aaron Robinson <arobins@microsoft.com>

Co-authored-by: Aaron Robinson <arobins@microsoft.com>

CollectionEqual ended up in infinite recursion as the array variant
no longer accepts the third string argument (there's no use for it
as Assert.Equal no longer supports it; after all, a single test was
using it) so that the enumerable variant ended up calling itself
instead of the array variant. I have removed the message string
and I deleted it from the one ResolveUnmanagedDllTests source.

In ThrowsWithInnerException there was a typo, there should be
"is TInner" in the inner exception check, not "is T".

Thanks

Tomas