PHPUnit: Add Database tags

PHPUnit:  Add Database tags

Several unit tests access the database, but did not declare
themselves in the Database @group.  This causes
"make databaseless" to fail needlessly.  Add the
missing tags.

tests/phpunit/includes/LinkerTest.php
tests/phpunit/includes/actions/ActionTest.php
tests/phpunit/includes/specials/ImageListPagerTest.php
tests/phpunit/includes/specials/SpecialMIMESearchTest.php
tests/phpunit/includes/title/MediaWikiPageLinkRendererTest.php

Other than ActionTest.php, these also are problematic in 1.23

Change-Id: I7c1c957e2194c13e48b7ba68d7529e5d89901875