Fixes for mocking in isolation

All existing Mock tests now work regardless of whether -UseModuleScope is specified.  Because these don't rely on implementation details, I've moved the Mock.Tests.ps1 file to the ScopeTests folder.
Merge branch 'Isolation' of https://github.com/dlwyatt/Pester into Isolation

Conflicts:
	ScopeTests/ModuleScope.Tests.ps1