Run module scoped and caller scoped tests in one run PoC

There are two motivations for this change:
- Exposing the UseModuleScope parameter to end users is imho not
necessary
- Pester is integrated with TC which assumes there is only one output
XML from the test run

This is only PoC to show my point by changing as little as I can. In
final version  I propose to differentiate the files by using .Tests.ps1
and .ModuleScopedTests.ps1