Search for more test locations in PHPUnitTestEngine

Summary:
See https://github.com/facebook/arcanist/pull/53

• Work correctly for directories with % in their name; this breaks under sprintf().
• Search for src/ -> tests/ style directories.
• Add coverage for search paths.

Test Plan:
Ran unit tests.

I don't have a working test case for PHPUnit tests, can one of you guys apply this and verify I didn't break your setups?

Reviewers: quard, aurelijus

Reviewed By: aurelijus

CC: aran

Differential Revision: https://secure.phabricator.com/D3558