Requirements: PHP7 in CLI mode
Run `arc unit` on a directory that has no tests
Result:
```
[2015-10-26 16:39:04] ERROR 2: Invalid argument supplied for foreach() at [/derpa/arcanist/src/unit/engine/PhutilUnitTestEngine.php:161]
arcanist(head=master, ref.master=dfde57ff812b), phutil(head=master, ref.master=db3667a62f66)
#0 PhutilUnitTestEngine::getTestPaths() called at [<arcanist>/src/unit/engine/PhutilUnitTestEngine.php:129]
#1 PhutilUnitTestEngine::getTestsForPaths() called at [<arcanist>/src/unit/engine/PhutilUnitTestEngine.php:20]
#2 PhutilUnitTestEngine::run() called at [<arcanist>/src/unit/engine/ArcanistConfigurationDrivenUnitTestEngine.php:143]
#3 ArcanistConfigurationDrivenUnitTestEngine::run() called at [<arcanist>/src/workflow/ArcanistUnitWorkflow.php:175]
#4 ArcanistUnitWorkflow::run() called at [<arcanist>/scripts/arcanist.php:382]
No tests to run.
```
We probably shouldn't do that