Redmine #5380: Introduce privilege elevation in Windows testing.

This is needed in order to run tests as a user other than
Administrator, even if that user has admin privileges. The reason is
that Windows by default runs every process with non-admin rights, so
we need to elevate the privileges ourselves.
Merge pull request #1664 from kacfengine/gainroot_for_windows

Redmine #5380: Introduce privilege elevation in Windows testing.