run-tests: Fix getting stuck when the test process dies uncleanly

When a test process spawns a subprocess that keeps STDOUT open and then dies
uncleanly, while writing something to STDERR `run-tests.php` would erroneously
attempt to read from `STDOUT` and then get blocked until the subprocess dies
(which might never happen).

Author: TimWolla

ext/zend_test/tests/run-tests_stuck_when_subprocess_exists.phpt

@@ -0,0 +1,16 @@
+--TEST--
+run-tests: Does not get stuck when a test spawns a subprocess and dies uncleanly.
+--EXTENSIONS--
+posix
+--FILE--
+<?php
+
+$handle = popen("sleep 5; echo not visible", "r");
+
+fwrite(STDERR, "foo\n");
+posix_kill(posix_getpid(), 9);
+
+?>
+--EXPECTF--
+foo%A
+Termsig=9

run-tests.php

@@ -1229,10 +1229,10 @@ function system_with_timeout(
         }
 
         if ($n > 0) {
-            if ($captureStdOut) {
-                $line = fread($pipes[1], 8192);
-            } elseif ($captureStdErr) {
-                $line = fread($pipes[2], 8192);
+            if ($captureStdOut && isset($r[1])) {
+                $line = fread($r[1], 8192);
+            } elseif ($captureStdErr && isset($r[2])) {
+                $line = fread($r[2], 8192);
             } else {
                 $line = '';
             }