Make TestUser way faster

By only updating the user row in the database, if needed
and by making use of md5 for passwords instead of slower
hashing.
This cut down run time of some Wikibase API tests to 20%
of the original value for me!
Also it reduces the run time of MediaWiki's test suite
by about 45s on jenkins.

Change-Id: I7024b287a71fe9b327dbcdc5427cd8edb5047606
Merge "Make TestUser way faster"