Add support for "xfail" to phpt SKIPIF sections

If SKIPIF output starts with "xfail" instead of "skip", the test
will be marked as XFAIL with the following message. Example:

    --TEST--
    Test xfailif feature
    --SKIPIF--
    <?php
    if (some_cond()) die('xfail Expected to fail!');
    ?>
    --FILE--
    ...

Author: nikic

run-tests.php

@@ -1590,6 +1590,11 @@ function run_test($php, $file, $env)
 					$info = " (warn: $m[1])";
 				}
 			}
+
+			if (!strncasecmp('xfail', ltrim($output), 5)) {
+				// Pretend we have an XFAIL section
+				$section_text['XFAIL'] = trim(substr(ltrim($output), 5));
+			}
 		}
 	}