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 5d26a6cbc63c1d5f9fdd6941e3fce46302f7864c)