If I run 'arc unit' when on a system with a case insensitive filesystem (e.g. OSX or an NFS-mounted share from OSX to a linux VM), PhpunitTestEngine forks more processes than it needs to.
Output from "ps aux | grep ' php'":
```
vagrant 17433 0.0 1.0 195760 22244 pts/0 S+ 22:48 0:00 php /home/vagrant/arcanist/bin/../scripts/arcanist.php unit
vagrant 17458 49.5 7.8 335372 161352 pts/0 R+ 22:48 9:42 php vendor/bin/phpunit ... /home/vagrant/Web/src/[Path]/tests/[Path1]Test.php
vagrant 17460 49.6 6.5 309128 135180 pts/0 R+ 22:48 9:44 php vendor/bin/phpunit ... /home/vagrant/Web/src/[Path]/tests/[Path2]Test.php
vagrant 17463 49.5 6.5 309128 135184 pts/0 R+ 22:48 9:43 php vendor/bin/phpunit ... /home/vagrant/Web/src/[Path]/Tests/[Path2]Test.php
vagrant 17464 49.4 7.8 335364 161640 pts/0 R+ 22:48 9:41 php vendor/bin/phpunit ... /home/vagrant/Web/src/[Path]/Tests/[Path1]Test.php
```
As you can see, both of the $test_dir_names have resolved to files that 'exist' on the filesystem, however they are the same file.