Running `arc unit` with `PytestTestEngine` set in `.arcconfig` ends with exception:
```
(DomainException) Attempt to write to undeclared property PytestTestEngine::project_root. at [<phutil>/src/object/Phobject.php:30]
```
I was able to fix this by adding a property to class:
```
--- PytestTestEngineOrig.php 2015-07-21 12:10:21.010315159 +0200
+++ PytestTestEngine.php 2015-07-21 12:10:40.609450176 +0200
@@ -5,6 +5,8 @@
*/
final class PytestTestEngine extends ArcanistUnitTestEngine {
+ private $project_root;
+
public function run() {
$working_copy = $this->getWorkingCopy();
$this->project_root = $working_copy->getProjectRoot();
```