librados: fix pg command test

Stat a bunch of (non-existent) random objects in the pool so ensure the
pg exists on the OSD before we assert that we get a 0 from querying it.

Although it is somewhat tempting to make the pg commands block until the
pg exists, that defeats much of the value of the command as a diagnostic
tool as it could block indefinitely instead of informing the admin/dev
that "the pg isn't there yet".

In any case, this fixes the api test failure.

Signed-off-by: Sage Weil <sage@inktank.com>