mgr/dashboard: Update PG update test

mgr/dashboard: Update PG update test

It is now commented out like it was before,
but I've added a comment what happened during this test with the QA
system. The problem was that even with only a increase of 1 PG the QA
cluster went into a cluster warning state and did not recover in time.
The QA coverage timeout is 2 minutes.

I could not reproduce this behavior with a local cluster, but I've
added a loop to wait until pgp and pg number are equal and the cluster
is in a healthy state again. This can take locally about 5 seconds.
The internal loop has a timeout of 3 minutes.

Fixes: https://tracker.ceph.com/issues/36362
Signed-off-by: Stephan Müller <smueller@suse.com>