Removed duplicate test

While checking out #328, I noticed a test may have been inadvertently copy/pasted:

"store.createRecord(type) returns a record with a container"

https://github.com/ebryn/ember-model/blob/master/packages/ember-model/tests/store_test.js#L55
https://github.com/ebryn/ember-model/blob/master/packages/ember-model/tests/store_test.js#L60