Setup caching mechanism based on test generator being used.

[rwjblue]

Since the generated linting files are actually the emitted output of the provided test generator, we need to ensure to entangle the tree generated by the lintTree hook with the currently in use test generator.

This will require some coordination amongst ember-cli-qunit and ember-cli-mocha (as well as a similar update in ember-cli-eslint).

[rwjblue]

I believe that we should hang a .baseDir() and .cacheKey() off of the test generator function (which is setup in ember-cli-qunit and ember-cli-mocha), and then use that for the generated lint tree.

[Turbo87]

FWIW porting ember-cli/ember-cli-eslint#176 to this addon should solve this issue too