PHPUnit now recognizes extension parser tests

Parser tests are registeredd by appending one or more .txt files to the
$wgParserTestFiles global setting.  Since this is shared with MediaWiki
core, I have made MediaWikiParserTest a factory of PHPUnit testsuite
which would filter in/out extensions tests.

The `extensions` test suite now has a second test suite builder which is
simply a wrapper around MediaWikiParserTest factory.

Play cases:

  $ php phpunit.php --group Parser --tap

Runs any parser tests including the ones coming from extensions.

With an extension having parser tests such as Cite:

  $ php phpunit.php --testsuite extensions --tap
  // Extensions tests are run including parser tests.

bug: 42506
Change-Id: Icc3e9d30706b32149aa9dd18552e4241ec4af67e
Merge "PHPUnit now recognizes extension parser tests"