setup tests: mock UAC status

This allows testing setup behavior both with UAC enabled and disabled.

The "mocking" mechanism actually changes the UAC registry setting, which
is read by UAC checking code in Chocolatey. Since the change requires OS
restart to take effect, it is indeed a mockup. However, the test suite
should not be interrupted, so that the setting is changed back to the
original value.

No effort has been made to support running the tests on XP. A modern OS is
assumed.