debuginfod testing: SIGUSR2 vs "groom" metric synch

Previous code did not account for a groom job that was already
completed at startup, so the SIGUSR2-triggered one may not have
completed in time for the test.  The shell won the race condition on
most buildbot VMs, but a debian builder showed the error of our ways.

Signed-off-by: Frank Ch. Eigler <fche@redhat.com>