Add mechanism to skip the skipif cache

If the SKIPIF section return nocache, don't cache the result. This
is used by tests that perform complex test set in conjunction with
skip checks, and need to be evaluated every time.

Author: nikic

ext/mysqli/tests/mysqli_pam_sha256.phpt

@@ -75,6 +75,7 @@ if (!$link->query(sprintf("GRANT SELECT ON TABLE %s.test TO shatest@'%%'", $db))
 }
 
 $link->close();
+echo "nocache";
 ?>
 --FILE--
 <?php

ext/mysqli/tests/mysqli_pam_sha256_public_key_ini.phpt

@@ -88,6 +88,7 @@ if (!$link->query(sprintf("GRANT SELECT ON TABLE %s.test TO shatest@'%%'", $db))
 }
 
 $link->close();
+echo "nocache";
 ?>
 --INI--
 mysqlnd.sha256_server_public_key="test_sha256_ini"

ext/mysqli/tests/mysqli_pam_sha256_public_key_option.phpt

@@ -85,6 +85,7 @@ if (!$link->query(sprintf("GRANT SELECT ON TABLE %s.test TO shatest@'%%'", $db))
 }
 
 $link->close();
+echo "nocache";
 ?>
 --FILE--
 <?php

ext/mysqli/tests/mysqli_pam_sha256_public_key_option_invalid.phpt

@@ -85,6 +85,7 @@ if (!$link->query(sprintf("GRANT SELECT ON TABLE %s.test TO shatest@'%%'", $db))
 }
 
 $link->close();
+echo "nocache";
 ?>
 --FILE--
 <?php

run-tests.php

@@ -3749,7 +3749,9 @@ public function checkSkip(string $php, string $code, string $checkFile, string $
 
         save_text($checkFile, $code, $tempFile);
         $result = trim(system_with_timeout("$php \"$checkFile\"", $env));
-        $this->skips[$key][$code] = $result;
+        if (strpos($result, 'nocache') === false) {
+            $this->skips[$key][$code] = $result;
+        }
         $this->misses++;
 
         if (!$this->keepFile) {