Run tests sequentially, rather than in parallel (#464)
Run tests sequentially, rather than in parallel (#464)

By default, jest runs the tests in parallel, which causes
us all kinds of grief for things that touch the filesystem.

An example of the kind of problem is described in #460

Currently, the tests take about 8 or 9 seconds to run when run in
parallel, and about 17 seconds to run when run sequentially, so
I'm going to suggest that the extra time isn't really a concern.

This also becomes more of an issue when testing addons, which
requires modifying files on disk.
mozilla-iot/gateway,moziot/gateway,mozilla-iot/gateway,moziot/gateway,mozilla-iot/gateway,mozilla-iot/gateway,mozilla-iot/gateway,moziot/gateway