Convert the make tests to use drush's PHPUnit framework

Initial work on this has been done. The test makefiles have been moved into a tests/makefiles directory.

Related issue: #1325910: Speeding up the make tests

All tests are now passing (with the patch from #1331392: Allow calls to Drush_CommandTestCase::drush to pass in the expected exit code), except for the d.o.-specific plugin tests, which are currently commented out. These remaining tests that need to be ported are a bit trickier than the simple build tests.

Tests for d.o-specific functionality have been moved over to the new d.o drush project #1365484: Move drupal.org-specific functionality to drupalorg_drush project. However, I'm going to refactor the make tests so that each makefile gets its own test. As it is now, phpunit chugs along until it encounters a failure, then stops running the rest. Moving them into separate tests will allow people to quickly see how many makefiles are passing or failing.

I just pushed up a commit that moves from a single test to 15 separate tests, so a single failure won't stop the rest of the make tests from running. AFAIK, these tests are now fully ported.