Clean up the "checkds" system test

Clean up the "checkds" system test

The "checkds" system test contains a lot of duplicated code despite
carrying out the same set of actions for every tested scenario
(zone_check() → wait for logs to appear → keystate_check()).  Extract
the parts of the code shared between all tests into a new function,
test_checkds(), and use pytest's test parametrization capabilities to
pass distinct sets of test parameters to this new function, in an
attempt to cleanly separate the fixed parts of this system test from the
variable ones.  Replace format() calls with f-strings.