test: correctly mask supporting services in tests

It turns out the "supporting services" were run in _all_ tests if
TEST-01-BASIC was run as the first test (which is usually the case),
since with the original condition in test_create_image() we would skip
the masking and then propagate the change to the default image used by
other tests. This has been causing multiple bogus test timeouts
(especially when the hwdb was being rebuilt in tests with short
timeouts, like TEST-52-HONORFIRSTSHUTDOWN).

Let's "fix" this by making the call to mask_supporting_services()
uncoditional and override the test_create_image() function in
TEST-01-BASIC to avoid the masking in this single case.