For simplicity, this implements just on in `NO_TASKS` (the default) or
on in `ALL_TASKS` (opt-in). This disables all category registrations
when in background task mode; we'll selectively re-enable things as
appropriate.
The flag constants were chosen to smoothly extend to a (16-)bit set in
the future, should we want to add a `JUST_TASKS("task", "other-task")`
option in the future.
This also adds ython tests for gen_static_components.py exercising
categories, simply 'cuz it's easiest to see what this adds in such
tests. Functional tests will follow in patches that actually
implement the new background tasks functionality.
Differential Revision: https://phabricator.services.mozilla.com/D96654
3d0d39d82c