This change reduces the VM used and memory allocated in the emit
tests by moving the WinRT tests to their own DLL and removing
the extra mscorlib reference.

The cause of most of the allocations in the Emit unit tests was
serializing the WinRT assemblies across app domains. There were
three reasons the DLLs needed to be serialized:

1) PeVerify currently requires a separate app domain to be used if
   any of the references of the assembly to be verified have already
   been loaded into the current app domain and have a different MVID.

2) The WinRT tests all use a separate version of mscorlib, so they
   will all reference an assembly with a different MVID than the one
   loaded.

3) Most of the WinRT tests were accidentally pulling two references
   to mscorlib, also always leading to a conflict.

All of these issues should be resolved. The CSharp.Emit unit tests
now allocated 400 MB less and have > 100 MB smaller peak VM.

Fixes #386.