[9.18] fix: test: Allow re-run of mkeys system test

On some slow systems, the test might intermittently fail due to inherent
timing issues. In our CI, this most often happens in the
system:gcc:8fips:amd64 jobs.

Closes #3098

Backport of MR !9732

Merge branch 'backport-3098-allow-re-run-of-mkeys-test-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!9765