karma-sinon-chai is not compatible with Chai 5+, and Karma is no longer
being updated.
Load sinon and chai manually instead, until we can have a long term plan
in place.
We want tests to be independent, so we cannot have them modify a shared
state, such as the l10n singleton. Make sure each test instantiates its
own object instead.
We should not be listing this in LINGUAS as that gives the impression
that English has en explicit translation. Instead, it is a special case
that the code needs to be explicitly aware of.
This reverts 9a06058 in favour of a more robust fix.
