Document how to run the tests, in particularly a subset (if relevant)

[mcomella]

Like #620, it isn't always obvious to new devs how they can run the tests: it'd be a good to document it. It's also helpful to know how to a run a subset of the tests.

┆Issue is synchronized with this Jira Task

[csadilek]

Moved to bugzilla: https://bugzilla.mozilla.org/show_bug.cgi?id=1795078

Change performed by the Move to Bugzilla add-on.