test/librados/cmd.cc: tolerate thrashing on pg_command tests

We may get ENXIO (osd down) or ENOENT (pg dne (yet) on the target osd) if
there is thrashing going on.

Fixes: #6122
Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #542 from ceph/wip-6122

test/librados/cmd.cc: tolerate thrashing on pg_command tests