qa: Add regression test for client ctime update on matching atime

Orchestrates a multi-client contention scenario using two
mounts to explicitly downgrade client_a down to SHARED caps by reading
from client_b. It then executes a touch/setattr operation on client_a
with an atime value identical to the file's current cached atime.

Verifies that ctime is advanced appropriately by the MDS despite the
value-equality match, preventing future regressions where the client
might strip CEPH_SETATTR_ATIME from the network mask.

Fixes: https://tracker.ceph.com/issues/71639
Signed-off-by: Jos Collin <jcollin@redhat.com>