fix MessageCacheTest for non-english wikis

MessageCacheTest::addDBData is called before setUp, so wgLanguageCode
and wgContLang are not set to 'en', which result in mediawiki pages
created as $key/en. When the test runs, the language is set correctly
and mediawiki is searching under $key, and does not find anything.

Set wgLanguageCode and wgContLang at the begin of addDBData

1) MessageCacheTest::testMessageFallbacks with data set #0
('FallbackLanguageTest-Full', 'ab', 'ab')
Message fallback failed.
Failed asserting that false matches expected 'ab'.

Change-Id: I8e70b7b424df6a9ad98bfb6c228809f4b4437d1e