Tests overhaul

Moved all tests into either a ModuleScopedTests or CallerScopedTests directory, based on whether the tests are focused on Pester internal functions and must be called with Invoke-Pester -UseModuleScope.  As many tests as possible have been moved out into the CallerScoped directory, since that's what tests the end-user experience.

Removed all of the lines that would dot-source the SUT files, since these are not necessary when testing Pester itself.  (Everything is already imported when the module is loaded.)