rados/singleton-nomsgr: add multi-backfill reservation rejection test

This triggers #7922.

Signed-off-by: Sage Weil <sage@inktank.com>
rados/thrash: rejection backfill reservations sometimes while thrashing
Merge pull request #24 from ceph/wip-7922

rados/singleton-nomsgr: add multi-backfill reservation rejection test