engine: fix RunVmCommandTest

OsRepository mocking was missing from the test, while the code was built
on the assumption that it is created and configured already. This test
failed in unlucky (but not impossible) situations when it ran before any
other tests that could have initialized the OsRepository.

Change-Id: I6c7b8935273b91da8d1c06eb9c9867212ca022c1
Signed-off-by: Laszlo Hornyak <lhornyak@redhat.com>