qa/standalone: Add missing teardowns to a subset of osd tests

qa/standalone: Add missing teardowns to a subset of osd tests

The following files and tests in them did not teardown the
cluster after a test completed.

1. osd/osd-force-create.sh
2. osd/osd-reuse-id.sh
3. osd/pg-split-merge.sh

This wouldn't cause issues if the tests are run individually. But when
running all the tests in the files mentioned above, it could introduce
unexpected test failures down the line. For e.g., multiple tests may
create pools with same name and if they are not cleaned up properly, this
could result in unexpected failures in a subsequent test.

Fixes: https://tracker.ceph.com/issues/51580
Signed-off-by: Sridhar Seshasayee <sseshasa@redhat.com>