Add UnmanagedCallersOnly runtime testing

We don't have .NET 5 ref assemblies yet, so I can't remove the override flags/custom definition of UnmanagedCallersOnlyAttribute. But when running on CoreCLR, everything does work when overriding our error, so I'm now testing that. As part of this, I rewrote the existing Thiscall tests to use pure C#, instead of a ton of IL. This was a line-by-line port of the existing IL. I then wrote a UnmanagedCallersOnly equivalent of the simple Thiscall case. While I did convert ThiscallBlittable to pure C#, I don't believe that it exercises anything different from a C# perspective so I didn't add an UnmanagedCallersOnly version of it.