Follows-up T349580: Add Less.js v2.5.3 spec to less.php test suite.

Scope

• Add the specification files from Less.js 3.13.1 ("less", "data", and "css" directory), and register them in phpunit/FixturesTest.php and in test/fixtures.php for test/compare.php.
• Document current failures in Phab tasks, and attach to parent task T288498.
• Disable current failures via the KNOWN_FAILURE constant in CI so that we can work on them one at a time.