Add unit test mode to srdnsupdater

This makes it not send any messages to srdns, thereby not changing the
dns server. It logs everything, though.
Fix tcpdump in runner

Had carried over a spurious stoneridge.outdir from the merge in from
master. Replaced with a reference to get_config('run', 'out')
Fix running xpcshell

Had missed a reference to an old global variable that now needs to be
determined at runtime in run_spcshell (specifically, the xpcshell
variable). This fixes that, as well as adds some logging of the
xpcshell command line being run.
Fix incorrect config variable

Had renamed stoneridge.tests -> stoneridge.testroot, but forgot to
change that in the runner.
Add unit test support in the runner