use ember-exam to speed up test runs

[runspired]

ember-exam allows for load balancing and parallelizing test runs. I've configured this to balance across 16 partitions to maximize cores used.

Example with:

Out of requested 16 browser(s), 16 browser(s) has launched.

1..1139
# tests 1139
# pass  1137
# skip  2
# todo  0
# fail  0

# ok
✨  Done in 159.42s.

Example without:

1..1124
# tests 1124
# pass  1122
# skip  2
# todo  0
# fail  0

# ok
✨  Done in 311.09s.