test/librados/testcase_cxx: set_allow_ec_overwrites wait for overwrite to work

test/librados/testcase_cxx: set_allow_ec_overwrites wait for overwrite to work

The mon command to enable ec overwrites succeeds once the mon commits
the update.  However, actual IOs won't suceed until the map is actually
propagated to OSDs.  Update set_allow_ec_overwrites to try performing
overwrites until it actually succeeds (with a 120s timelimit) before
returning.

Fixes: https://tracker.ceph.com/issues/66226
Signed-off-by: Samuel Just <sjust@redhat.com>
(cherry picked from commit 426f225d4e6ec7e037260a234896ef9c8d6aec82)