Don't use deprecated User::checkPassword() in TestUser

Don't use deprecated User::checkPassword() in TestUser

I423f09f added this call in an attempt to speed things up. However,
User::checkPassword() was deprecated in I2c736ad7, and the call causes
some tests to fail when $wgDisableAuthManager is false.

Since this is trying to determine whether the user_password column in
the user table needs updating for the unit test, let's test that
directly instead of the much-more-complicated User::checkPassword().

Change-Id: I410de706f9074edea3f3988769a3e99231d8dca3