process_parallel was leaking the failures on its internal deferreds.  closes #458

DeferredList implemented cancellation in Twisted 13.2.0 by holding a
reference to the affected deferreds objects, if a deferred errored the
result was propagated to the DeferredList but still referenced by the
original deferred and nobody was consuming it.

The tests started to fail because the reference from DeferredList
prevented the underlining deferred from been collected before the test
finish invalidating the effect of self.flushedLoggedErrors() call.