This moves testing/profiles/prefs_general.js to
testing/profiles/common/user.js. It also adds an 'extensions' folder to the
common profile. Dropping extension files here will get them installed in all
test harnesses (useful for testing on try).
The idea is that all test harnesses will eventually use this 'common' profile.
We'll also create some new per harness profiles, e.g testing/profiles/mochitest
and testing/profiles/reftest. This way there will be a single location
developers can go to set preferences, both for a specific harness, and across
all harnesses.
MozReview-Commit-ID: 8sqBqLiypgU
5f240d3ef3