Assembly.ReflectionOnlyLoad should throw PNSE

[jkotas]

Reflection-only loads are not supported in CoreCLR. (FEATURE_REFLECTION_ONLY_LOAD)

Assembly.ReflectionOnlyLoad are doing full loads instead of reflection-only loads. They should throw PNSE instead.

The matching corefx tests should be updated: https://github.com/dotnet/corefx/blob/f9efc9625f795673c5c17d0791d8c79710a267b9/src/System.Runtime/tests/System/Reflection/AssemblyTests.cs#L161

[jkotas]

Note that other ReflectionOnly APIs like AppDomain.ReflectionOnlyGetAssemblies() are doing the right thing already.

[jkotas]

ReflectionOnlyGetType also - we do not seem to have any tests for it.

[rahku]

Fixed