These tests insert variations of fake lint errors into the database, and
then read out of the database to check they round-trip properly.
And while we're at it, improve the setForPage() return value.
These tests can be run with something like:
php tests/phpunit/phpunit.php extensions/Linter/tests/phpunit/
Change-Id: Ifdba8a8a104d218a822f909bc5d7b3512aca499d
