Make the nocache tag actually work

If nocache is used, then we should return an empty string result,
so the test doesn't get marked as borked.

Also limit to an occurrence at the start, just like other skip
tags.

Author: nikic

run-tests.php

@@ -3749,8 +3749,10 @@ public function checkSkip(string $php, string $code, string $checkFile, string $
 
         save_text($checkFile, $code, $tempFile);
         $result = trim(system_with_timeout("$php \"$checkFile\"", $env));
-        if (strpos($result, 'nocache') === false) {
+        if (strpos($result, 'nocache') !== 0) {
             $this->skips[$key][$code] = $result;
+        } else {
+            $result = '';
         }
         $this->misses++;