Permit use of User::getDefaultOptions() in-process cache in unit tests

Permit use of User::getDefaultOptions() in-process cache in unit tests

MediaWiki uses an in-process cache to speed up repeat calls to
User::getDefaultOptions() -- except when the unit tests are running, in which
case the process cache is disabled, because otherwise it would be at risk of
becoming stale due to unit tests manipulating $wgContLang. Well, there's a less
aggressive option, which is to keep the cache enabled but use it only if
$wgContLang hasn't changed. Since MediaWiki's test setup code creates default
users for the unit tests, User::getDefaultOptions() ends up getting called
quite a lot, so enabling the process cache is worth the trouble.

Change-Id: I81f3ae42d356939c81e59ab12d7a9e7d1206cb40