[Feature] Add MU support for model and model-test blueprints

[dcyriller]

This PR aims at providing MU support for this ember-cli command: ember generate model foo.

It is a part of this quest issue: ember-cli/ember-cli#7530

Heavily inspired by #5476

[dcyriller]

In CI, ember-canary scenario is failing. It seems related to an upgrade of ember-canary version as every recent PR is failing on this test:

112) [Chrome 67.0] integration/adapter/build-url-mixin - BuildURLMixin with RESTAdapter: buildURL - buildURL takes the records from findMany
     Died on test #1     at Module.callback (http://localhost:7357/assets/tests.js:967:19)
    at Module.exports (http://localhost:7357/assets/vendor.js:111:32)
    at requireModule (http://localhost:7357/assets/vendor.js:32:18)
    at TestLoader.<anonymous> (http://localhost:7357/assets/test-support.js:10412:11)
    at TestLoader.require (http://localhost:7357/assets/test-support.js:10402:27)
    at TestLoader.loadModules (http://localhost:7357/assets/test-support.js:10394:16)
    at loadTests (http://localhost:7357/assets/test-support.js:11447:22): snapshots.get is not a function
     TypeError: snapshots.get is not a function
         at Class.adapter.buildURL (http://localhost:7357/assets/tests.js:973:38)
         at Class.findMany (http://localhost:7357/assets/vendor.js:89663:22)
         at _findMany (http://localhost:7357/assets/vendor.js:83754:27)
         at http://localhost:7357/assets/vendor.js:85576:15
         at Class._flushPendingFetchForType (http://localhost:7357/assets/vendor.js:85581:15)
         at Map.forEach (<anonymous>)
         at MapWithDefault.forEach (http://localhost:7357/assets/vendor.js:74318:42)
         at Class.flushAllPendingFetches (http://localhost:7357/assets/vendor.js:85460:26)
         at invoke (http://localhost:7357/assets/vendor.js:37864:24)
         at Queue.flush (http://localhost:7357/assets/vendor.js:37784:25)

Not sure how to fix it though 😕

[runspired]

The canary test failure is a known issue; however, I recently had to skip the adapter tests this is inspired by because it turns out that the emberNew() test helper for blueprint generation does not yet have a way to turn on the module-unification flag. Once canary of ember-cli offers this ability I will happily help move this forward :)

cc @thoover @rwjblue for ember-cli cleanup

[dcyriller]

@runspired do you think that something like this would solve our issue?

It allows to do: emberNew({ isModuleUnification: true }).

[runspired]

@dcyriller yes! Thanks for taking that on :)

[dcyriller]

I'll update the PRs once it has been merged :)

[dcyriller]

@runspired this one should be ready too