Allow testing of production assets.

[rwjblue]

• 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 ember-cli/broccoli-asset-rev#9.

Closes #215.

[stefanpenner]

👍