tests: Rename file to end in Test.php

Though the hook registration was working fine, the run from
Jenkins was failing.

Jenkins runs like this:

 $ php tests/phpunit/phpunit.php extensions/Gadgets

Which bypasses UnitTestsList and uses auto-discovery within the
given directory of "*.Test.php" files.

Renamed to file to match PHPUnit's internal discovery pattern
and updated the onUnitTestsList callback to use this pattern
as well.

Also updated documentation applied recent code conventions in
this area (extend MediaWikiTestCase).

Change-Id: Iaa52a5eafdebf1529dd02fb1bed89d27c298e0d3