Ensures force_clean doesn't run for pycc tests

There are tests that inherit from PyonTestCase but are part of the INT group of
integration tests. The PyonTestCase adds a file system cleanup at the
conclusion of the test runs. This patch does a quick env variable check to make
sure that a file system clean is allowed.
Merge pull request #507 from lukecampbell/fs

Ensures force_clean doesn't run for pycc tests