The default "yarn test" command runs ALL tests 3 times

# Problem

On my machine, when I run `yarn test`, or `yarn test --silent`, all tests execute 3 times. The desired behavior, since this actually executes `ember exam --split=3 --weighted --parallel` would be that 3 partitions would run, each running a fraction of the tests. In addition to that, the 3 partitions run consecutively, not in parallel.

This seems to be related to https://github.com/trentmwillis/ember-exam/issues/108 and, following the conversations there and adding `parallel: -1` to testem.js seems to cause the partitions to execute in paralel, but each partition still runs all of the tests in the suite.