Code inspection suggested that keys imported into the CNG MachineKey store
from PFXImportCertStore were not getting properly cleaned up.  This change
adds tests that prove that supposition, and then fixes the bug so they pass.

* Bump keysize to 2048
  * This caused the tests to be too slow, so reuse 6 random keys for all of them
* Remove the random ordering in machine-or-user for defaultkeyset (try both ways)
* Remove incorrect copy/paste comment
* Remove bad nullable annotation