Do not mark 'unit tests OK' in reviews when Unit Tests returned non-zero exit

[iamPHEN]

Currently the error code isn't being checked when unit tests are ran.

https://github.com/tagview/arcanist-extensions/blob/master/tap_test_engine/src/TAPTestEngine.php#L17

I have little understanding about PHP, but here's my hacky workaround:

    list($error, $stdout, $stderr) = $future->resolve();

    if ($error <> 0) {
      $msg = "not ok 0 - Unit test returned non-zero exit\n";
      echo $msg;
      return $this->parseOutput($msg);
    }