Skip the test in test_failing_integrals that is too slow for @slow

This test never finishes (or at least, it would probably take days), so
without a timeout, the slow tests don't complete. This was causing the slow
tests to timeout in the NiPy buildbot
(http://nipy.bic.berkeley.edu/builders/sympy-py2.6-32/builds/0/steps/shell_4/logs/stdio).

@matthew-brett @moorepants