Restore directory after executing ArcanistBundleTestCase

Summary:
This test currently chdir()'s into a directory which is later removed. If another test tries to run a shell script while the CWD is invalid, the shell may emit this to stderr:

  shell-init: error retrieving current directory: getcwd: cannot access parent directories: No such file or directory

Among other things, this can cause the XHPAST test to fail, because it detects syntax errors by examining stderr.

Instead, retore the directory.

Test Plan: Ran "arc unit --everything", which could previously fail if XHPAST ran after Bundle.

Reviewers: btrahan, vrana

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D4738