Being able to run tests by setting $PsModulePath instead of XCopy to one of those folders

[bergmeister]

As discussed internally. Instead of doing a Copy-Item of the PSScriptAnalyer folder in the out folder to the first $env:PSModulePath folder (where it can override an existing installation due to the version number being the same), we should just be able to just set add the out folder to the beginning of the $env:PSModulePath variables during test execution. However, this does not even work for Windows PowerShell at the moment due to the LibraryUsageTests (I get one failure here).