Bug 1667276 - Part 3: Load a custom prefs file when running a background task. r=mossop,KrisWright

There are some complications here to handle unpackaged and packaged
builds.  In addition, there could be a difference between App prefs
and GRE prefs.  Since the underlying backgroundtasks code is built as
part of Gecko (i.e., `toolkit/...` rather than `browser/...`) I have
favoured GRE prefs.  I think, however, that what is written will work
for App-specific prefs, but I'm not concerned with that detail at this
time.

This also add tests for backgroundtask-specific prefs, which are
structured as both xpcshell and mochitest-chrome tests because
locally, the former tests unpackaged builds and the latter can
accommodate testing packaged builds.  We could use mochitest-chrome
for both, but this has been pleasant to work with locally.

Differential Revision: https://phabricator.services.mozilla.com/D97510