qa/standalone/osd/pg-split-merge: fix import_after_merge_and_gap

qa/standalone/osd/pg-split-merge: fix import_after_merge_and_gap

This test introduces a map gap.  What *should* happen is that when there is
such a gap, we cannot import.  Previously, the test didn't reliably produce
a map gap at all, and didn't check that import failed--it verified that it
passed.

Fix the test so that it reliably produces a gap *and* reports
min_last_epoch_clean to the mon so we can trim.  Then verify we fail to
import, but can with --force.  But remove the pg again, because if we
force an import with a map gap the osd will refuse to start.

Fixes: http://tracker.ceph.com/issues/38525
Signed-off-by: Sage Weil <sage@redhat.com>