Various test fail when run in plain mode (check out @Krinkle patch I69f735eb56c1362189298d9859d3add576faaadb https://gerrit.wikimedia.org/r/#/c/216886/ to test)
Run > grunt qunit in the mediawiki core directory
Tests should be amended so this works.
The problematic tests are:
ext.uls Initial check FAILED: Unfinished AJAX request.
ext.uls User preferences FAILED: Options saving API did not produce an error. Expect: true. Actual: false.