Implement HTMLT tests, and migrate HTMLPurifierTest to this format.

HTMLT tests are a compact and easy-to-use way of making assertPurification
type tests. They take the format of:

--INI--
Ns.Directive = "directive value"
--HTML--
Input HTML
--EXPECT--
Expected HTML

Expect more features and migration to be coming soon.

Signed-off-by: Edward Z. Yang <edwardzyang@thewritingpot.com>

tests/common.php

@@ -161,6 +161,8 @@ function htmlpurifier_add_test($test, $test_file, $only_phpt = false) {
             return $test->add(path2class($test_file));
         case '.vtest':
             return $test->add(new HTMLPurifier_ConfigSchema_ValidatorTestCase($test_file));
+        case '.htmlt':
+            return $test->add(new HTMLPurifier_HTMLT($test_file));
         default:
             trigger_error("$test_file is an invalid file for testing", E_USER_ERROR);
     }