qa: make all replayer threads busy and query 'syncing' status

qa: make all replayer threads busy and query 'syncing' status

* Make all replayer threads busy and then query for 'syncing' state
instead of just fetching the current status.

* Dropped 'current_syncing_snap' check, as it's not compulsory for
this test. The actual intension is to make threads in 'syncing' status
and 'current_syncing_snap' check is not necessary for that.

* Drop 'snaps_deleted' metrics check in test_cephfs_mirror_cancel_mirroring_and_readd.
test_cephfs_mirror_cancel_mirroring_and_readd primarily focusses
on the synchronization of the newly added directory paths post removal
of the previously added/syncing directory paths. So checking of 'snaps_deleted'
metrics is unnecessary here.

* Wait for more time to finish the new snapshot creations and the sync backoff.
We need to wait for more time in test_cephfs_mirror_cancel_mirroring_and_readd,
as the test makes all replayer threads busy.

Fixes: https://tracker.ceph.com/issues/64711
Signed-off-by: Jos Collin <jcollin@redhat.com>