Clean tests step now removes stale coverage report files.

Added test cleaning to fasttest_* instead of test_*, so that
it always gets executed for lms and cms tests.

Added comment to `directory` command for tests.rake

Separated cleaning of test fixtures from cleaning of reports
directory to resolve conflicts with collectstatic.
Merge pull request #114 from edx/will/test-report-cleanup

Clean tests step now removes stale coverage report files.