Test cleanup
* Pulled out the Pester assertions into a separate file
* Pulled out the testing helper functions into a separate file
* Put a check in PsGet.tests.ps1 to write a friendly message the Run-Tests.ps1 must be used to execute tests
Test cleanup
* Added Pester tests for GetPsGet.ps1
* Change Global module path to use $env:CommonProgramFiles instead of $env:CommonProgramFiles(x86)
* Pass through EnableExit to allow the Invoke-AllTests.cmd to get failure exit code