Fix arc unit when running from non-root directory

Summary:
`$this->getPaths()` gives us paths relative to repository root.
We resolve them relative to cwd.

Test Plan: [src] `arc unit difference`

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Koolvin

Differential Revision: https://secure.phabricator.com/D2634

src/unit/engine/phutil/PhutilUnitTestEngine.php

@@ -48,7 +48,7 @@ final class PhutilUnitTestEngine extends ArcanistBaseUnitTestEngine {
           "      operation is not supported.");
       }
 
-      $path = Filesystem::resolvePath($path);
+      $path = Filesystem::resolvePath($path, dirname($library_root));
 
       if (!is_dir($path)) {
         $path = dirname($path);