We are starting to spin off more and more "variants" of test suites. These are
usually just duplicates of our pre-existing tasks, except with an additional
pref set.
Currently there are two variants (serviceworker-e10s and socketprocess-e10s),
but a third will be added soon (fission). This change ensures we handle these
types of requests in a consistent and well defined manner. It also splits tasks
in a loop, so we don't accidentally risk combinatorial explosion.
Variants should typically be reserved for very large changes that will impact
the entire codebase (think e10s).
Differential Revision: https://phabricator.services.mozilla.com/D28061
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extra : moz-landing-system : lando
e105223614