Migrate marionette tests because of deprecation
At some point we should migrate to something else, since
marionette_driver are deprecated: https://groups.google.com/forum/#!topic/mozilla.tools.marionette/s6_03Qt2ubc (also https://lists.mozilla.org/pipermail/tools-marionette/2019-January/000060.html). Given the effort required, I would try to postpone this as much as possible, so I would keep current
marionette as long as tests are reasonably in good shape.
If at some point tests start failing without good reason, we could try to use
marionette_harness from the in-tree gecko repository, since these will be more up-to-date than the pypi packages we currently use. Then, if that doesn't solve the issues, we should decide what's the best alternative and port the tests.