Use the Xunit assert library instead of CoreCLRTestLibrary for most asserts (#61226)

Use the Xunit assert library instead of CoreCLRTestLibrary for most asserts (#61226)

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.