Speed up password-handling in the unit tests

Speed up password-handling in the unit tests

* Speed up password generation and verification by setting MWOldPassword as the
  default password type. Do this once, in MediaWikiTestCase::makeTestConfig(),
  rather than in five different places.
* Rename '$pwhash' to '$passwordHash', for consistency. It's ugly to have both
  '$passwordFactory' and '$pwhash' in the same scope.
* Make TestUser::setPasswordForUser() check first whether the desired password
  is already set. This is actually the common case, since the password is reset
  in the setup code for every test, but only a few tests actually change the
  password.

Change-Id: I423f09ff7472b6cbde21cb709ea7c7ef9e298f18