Bug 1024660 - persistent failure in test_mutation for pop3:fake

At some point something changed in b2g's execution (ex: this did not used
to fail, but when I upgraded my b2g it now consistently fails) so that ugly
leftover state in _TEST_pendingHeaderDeletes from a test step that was not
relevant to POP3 was failing due to a refusal to structured clone the state.

I'm not sure exactly what the change was and I assert we really don't need
to care since we don't want to be persisting the state in question anyways.