tests: don't use SignalMixin

tests: don't use SignalMixin
It seems like we no longer need it, and it screws up something internal in
trial which causes trial's TestCase.mktemp() method to exhibit wrong behavior
(always using a certain test method name instead of using the current test
method name), and I wish to use TestCase.mktemp().

Of course, it is possible that the buildbot is about to tell me that we do
still require SignalMixin on some of our platforms...