ceph_test_rados_api_tier: try harder to trigger the flush vs try-flush race

It seems to be reasonable easy to complete a flush before the next client
request is processed.  Crazy...

Same with the flush vs write race.

Fixes: #7329
Signed-off-by: Sage Weil <sage@inktank.com>
Merge remote-tracking branch 'gh/wip-7329' into next