Add {TMP} placeholder for PHPT INI sections

Several tests use `/tmp` in the `--INI--` section, but this is not
portable.  We therefore introduce the `{TMP}` placeholder which
evaluates to the system's temporary directory using
`sys_get_temp_dir()`.

We also remove the doubtful `strpos()` optimization.

run-tests.php

@@ -1604,9 +1604,8 @@ function run_test($php, $file, $env)
 	// Any special ini settings
 	// these may overwrite the test defaults...
 	if (array_key_exists('INI', $section_text)) {
-		if (strpos($section_text['INI'], '{PWD}') !== false) {
-			$section_text['INI'] = str_replace('{PWD}', dirname($file), $section_text['INI']);
-		}
+		$section_text['INI'] = str_replace('{PWD}', dirname($file), $section_text['INI']);
+		$section_text['INI'] = str_replace('{TMP}', sys_get_temp_dir(), $section_text['INI']);
 		settings2array(preg_split( "/[\n\r]+/", $section_text['INI']), $ini_settings);
 	}