Add --hwtest option to gnome-shell-perf-helper

Add a --hwtest option to gnome-shell-perf-helper which runs the
tests in perf/hwtest.js with the appropriate environment, and then
logs the results using the 'gnome-hwtest-log' utility which is
available in the hardware testing environent.

(For development of hwtest.js in a normal environment, run the tests
as: gnome-shell-perf-tool --perf=hwtest --extra-filter=Gedit)

https://bugzilla.gnome.org/show_bug.cgi?id=732350