TEST-DEFERRED-WARNINGS *sometimes* fails on CMUCL

[Robert P. Goldman]

I get intermittent failures on TEST-DEFERRED-WARNINGS with CMUCL in the last two forms of the test:

(errors #+(or allegro clozure) compile-file-error
        #+(or cmucl scl) null
        #+sbcl compile-failed-error
        (let ((*compile-file-warnings-behaviour* :error))
          (load-system :undefined-variables)))
(errors #+(or allegro clozure) compile-file-error
        #+(or cmu scl) null
        #+sbcl compile-failed-error
        (let ((*compile-file-warnings-behaviour* :warning))
          (load-system :undefined-variables :force t)))

sometimes I get COMPILE-WARNED-ERROR in the first, sometimes nothing.

I suspect that this is because we aren't always appropriately cleaning out the filesystem for this test, so some fasls linger, leading to variance in the test results.