Fix pendulum.test to properly unwind after an exception

[njsmith]

Previously, if an exception occurred inside a 'with pendulum.test(...)'
block, then the monkeypatch would remain in place instead of being
un-done. This could cause confusing results, as one failing test could
cause other tests to run with an unexpected mock in place and cause
other failures.

[njsmith]

All the test failures appear to be unrelated.

[sdispater]

Thanks!