Merge branch '5.1' into 5.x

* 5.1:
  [Filesystem] Check if failed unlink was caused by permission denied
  fix APCu installation for the nightly build job
  skip Vulcain-based tests if the binary cannot be executed

Author: fabpot

Tests/HttpClientTestCase.php

@@ -302,6 +302,14 @@ private static function startVulcain(HttpClientInterface $client)
         sleep('\\' === \DIRECTORY_SEPARATOR ? 10 : 1);
 
         if (!$process->isRunning()) {
+            if ('\\' !== \DIRECTORY_SEPARATOR && 127 === $process->getExitCode()) {
+                self::markTestSkipped('vulcain binary is missing');
+            }
+
+            if ('\\' !== \DIRECTORY_SEPARATOR && 126 === $process->getExitCode()) {
+                self::markTestSkipped('vulcain binary is not executable');
+            }
+
             self::markTestSkipped((new ProcessFailedException($process))->getMessage());
         }