Allow concurrent `ember test --server` and `ember server`.

Prior to this, both `ember server` and `ember test --server` attempted
to write to (and clear) `tmp/output`. This allows the test task to use a
custom randomly named temp dir instead (which prevents the collision).
Merge pull request #862 from rjackson/ensure-random-directory-is-used-for-testem

Allow concurrent `ember test --server` and `ember server`.