With the Perl driver, machine.sleep(N) was doing a sleep on the guest
machine instead of the host machine. The new Python test driver however
uses time.sleep(), which instead sleeps on the host.

While this shouldn't make a difference most of the time, it *does*
however make a huge difference if the test machine is loaded and you're
sleeping for a minimum duration of eg. an animation.

I stumbled on this while porting most of all my tests to the new Python
test driver and particularily my video game tests failed on a fairly
loaded machine, whereas they don't with the Perl test driver.

Switching the sleep() method to sleep on the guest instead of the host
fixes this.

Signed-off-by: aszlig <aszlig@nix.build>

nixos/test-driver: Use guest time when using sleep