Tests runnable with arc unit

Summary:
Arcanist will now run PHPUnit tests. To comply with the tests
detection mechanism, we need the test file be called coreTest.php.

This change also simplify a little the Makefile.

Test Plan:
Launch tests with Make, Arcanist and PHPUnit:

• make test
• arc unit
• arc unit core.php
• phpunit .
• phpunit test/coreTest

Reviewers: dereckson

Differential Revision: http://devcentral.nasqueron.org/D85