Delete obsolete twig.contains tests
Move twig.range tests into Mocha suite
Move twig.spaceless tests into the Mocha suite
Port twig.forEach tests to the Mocha suite
Port twig.extend tests to the Mocha suite
Port twig.empty tests to the Mocha suite
Port twig.pregQuote tests to the Mocha suite

This is the last of twig_tests.html, so it's removed now.
Port the twig.Markup tests to the Mocha suite

This obsoletes markup_test.html, so that's gone too.
Port twig.Environment tests to the Mocha suite

All the tests in environment_test.html are ported, so that's another file gone!