acessor for MediaWikiTestCase::$useTemporaryTables

useTemporaryTables is marked private to prevent childs from overriding
the value which would cause a lot of madness whenever it forgot to reset
its status.  But that in turns prevents a child class from figuring out
whether we use temporary tables or not. The ActiveAbstract extension
test suite has such a requirement and hence need a proper accessor.

MediaWikiTestCase::usesTemporaryTables() is a public accessor to for the
private property MediaWikiTestCase::$useTemporaryTables.

Change-Id: I8016c49116a187d7523cc0d6190514f8a513eafb