I have some code to manage the default profile specifically for testing in the commits Bug 1703318. I'd like it to not wait for the metrics pieces, which will take a while to get hammered into shape, so I'm going to split that work into this ticket, which can land first.

We duplicate these functions in two head.js files right now, and I'm
loathe to add a third, so let's extract a shared test module.

This is simply for testing convenience: it's awkward to use the
profile service to set the default profile. Centralizing these
functions in one place both makes the utility functions consistent and
helps make future tasks that want to do similar things adopt the
testing conventions automatically.

When no default profile exists, or there is an issue reading data from
it, we have limited options to surface the issue. Since we can't read
preferences, we can't rely on driving loglevels up. Similarly, we
can't use Glean to report metrics since the user's datareporting
preferences won't be respected.

Returning exit codes will allow us to distinguish various situations
in the Windows tasksched.exe history at least, as well as allowing
us to test easily.

Depends on D111347

https://hg.mozilla.org/mozilla-central/rev/5b7500d84394
https://hg.mozilla.org/mozilla-central/rev/33f5ffa72023
https://hg.mozilla.org/mozilla-central/rev/baf49776c4b2