ember test --path does not set EMBER_ENV

[chancancode]

While working on emberjs/ember-inspector#1069, @rwjblue noticed that ember test --path ... does not set EMBER_ENV to test.

This means addons that relies on this will behave incorrectly. One example is the testem middleware.

This is only a problem for --path because it does not run a build, which usually sets the variable here.