Clean and repair many phpunit tests (+ fix implied configuration)

Clean and repair many phpunit tests (+ fix implied configuration)

This commit depends on the introduction of
MediaWikiTestCase::setMwGlobals in change Iccf6ea81f4.

Various tests already set their globals, but forgot to restore
them afterwards, or forgot to call the parent setUp, tearDown...

Either way they won't have to anymore with setMwGlobals.

Consistent use of function characteristics:
* protected function setUp
* protected function tearDown
* public static function (provide..)

(Matching the function signature with PHPUnit/Framework/TestCase.php)

Replaces:
 * public function (setUp|tearDown)\(
 * protected function $1(

 * \tfunction (setUp|tearDown)\(
 * \tprotected function $1(

 * \tfunction (data|provide)\(
 * \tpublic static function $1\(

Also renamed a few "data#", "provider#" and "provides#" functions
to "provide#" for consistency. This also removes confusion where
the /media tests had a few private methods called dataFile(),
which were sometimes expected to be data providers.

Fixes:

TimestampTest often failed due to a previous test setting a
different language (it tests "1 hour ago" so need to make sure
it is set to English).

MWNamespaceTest became a lot cleaner now that it executes with
a known context. Though the now-redundant code that was removed
didn't work anyway because wgContentNamespaces isn't keyed by
namespace id, it had them was values...

FileBackendTest:
* Fixed: "PHP Fatal: Using $this when not in object context"

HttpTest
* Added comment about:
  "PHP Fatal: Call to protected MWHttpRequest::__construct()"
  (too much unrelated code to fix in this commit)

ExternalStoreTest
* Add an assertTrue as well, without it the test is useless
  because regardless of whether wgExternalStores is true or false
  it only uses it if it is an array.

Change-Id: I9d2b148e57bada64afeb7d5a99bec0e58f8e1561