Random directory for TestDrive.

Test drive is set to a random directory. Instead of "Pester" a GUID with check for collision is used as the directory name. Running two Pesters in parallel is possible if started from two separate powershell.exe processes.
The directory is still placed in the $env:temp (user temp). The global $TestDrive variable and the global PSDrive TestDrive are still published. So it is not possible to run two Pesters in the same session at the same time.