setup: use "trial" executable instead of the setuptools_trial plugin for "make quicktest"

setup: use "trial" executable instead of the setuptools_trial plugin for "make quicktest"
This is to fix #591 ("make quicktest" could be quicker and less noisy).  This means that "make quicktest" won't work unless the user has manually installed Twisted already such that the "trial" executable is on their PATH and the Twisted package is on their PYTHONPATH.  This bypasses the behavior of setuptools_trial which builds and checks package dependencies before executing the tests.