test: add a test for join with self delete

Deletion of the own entry from _cluster space is allowed during
the join stage, because the remote master could have already had
the joining instance UUID in _cluster space but then deleted it.

Then for the joining instance it looks like deletion of self from
_cluster. But that is fine - in the end of join the master will
register the replica again.

The case is handled, but not covered with a test. The patch adds
one.

NO_DOC=test
NO_CHANGELOG=test