Fix HtmlTest, XmlTest; Add setMwGlobals method to base TestCase.

Fix HtmlTest, XmlTest; Add setMwGlobals method to base TestCase.

The HTML and XML test now set the globals they depend on
(instead of relying on the default settings).

Tests for the "other" scenarios still exist, globals are
overridden inline. They are automatically restored after each
test## function by PHPUnit from MediaWikiTestCase::tearDown.

Also fixed 2 other problems with the test suite:

* HtmlTest::testDropDefaults forgot to pass
  $message to the assertion from the provider ($case[3]).

* The data provider for HtmlTest::testDropDefaults was calling
  Html::element directly (instead of calling it within the test)
  which is problematic because data providers are expected to be
  static. PHPUnit calls them outside the setUp/tearDown flow.
  (also fixed the function to be public static, as PHPUnit
  expects).

That last one was crucial to make the test still pass correctly.

Updated the expected strings to what they are with these
fixed non-leakage settings.

Took wgHtml5 without xmlform as default. And added tests
for variations where it made sense.

Change-Id: Iccf6ea81f4bc2639273ab2ad101c58788ee49d45