Allow testing of production assets.

* Allow `--environment` to be used in `ember test` command.
* Cleanup `EmberApp` so that it uses `this.tests` instead of checking for `production` environment.
* Use `process.env.EMBER_CLI_TEST_COMMAND` to signify the current build was triggered by `ember test` (as opposed to `ember build` or `ember server`).
* Consolidate `process.env.EMBER_ENV` checking to a single location.

This is pending https://github.com/rickharrison/broccoli-asset-rev/pull/9.

Closes #215.