Fix TimestampTest broken by 7e3386d4

- Call RequestContext::getMain()->setLanguage() since
  MWTimestamp::getHumanTimestamp() will call getLanguage() to get the
  Language object (and it not always English depending on the order the
  tests are called)
- While I'm at it, use MediaWikiLangTestCase to avoid duplicating things

Change-Id: I910287ccf093ef5681cd09445501dccc8d3f4d14
Merge "Fix TimestampTest broken by 7e3386d4"