mon/test_mon_osdmap_prune: Use first_pinned instead of first_committed

mon/test_mon_osdmap_prune: Use first_pinned instead of first_committed

In the test, we assume the trimming of osdmaps is done if osdmap_first_committed is equal to the epoch of the map upto which we had to trim.
osdmap_first_committed however returns the value of cached_first_committed which is updated before the Paxos proposal for trim goes through.
Normally, this is not a problem as when a refresh() happens, cached_first_committed will be corrected to the value before the trim was proposed if the Paxos proposal failed.
But for this test, we encounter a failure in this case as the osdmap_first_committed will not be equal to the osdmap_manifest.first_pinned as the trim never went through.
Changing the test to verify that osdmap_manifest.first_pinned has changed to the correct value is a better way to signify that the trim is complete for the purpose of this test.

Fixes: https://tracker.ceph.com/issues/47838
Signed-off-by: Aishwarya Mathuria <amathuri@redhat.com>