iOS Unit Testing Temporary Directory Usage

Summary: Moves to the proper pattern of using PHP functions to locate the temporary directory. Replaces a hardcoded `/tmp` path. Also performs a bunch of linting for variable names and definitions.

Test Plan: Run `arc unit` on any iOS project, notice results returned the same as before.

Reviewers: jmillard

Reviewed By: jmillard

Differential Revision: https://devportal.imobile3.com/D3546