cleans up header notifications integration tests

Step #3 of the refactoring done here: https://github.com/discourse/discourse/pull/1615 and here: https://github.com/discourse/discourse/pull/1635.

Adds a single, focused integration test that verifies proper wiring of all the notifications dropdown parts and removes all other notifications dropdown related integration tests (that are now redundant after all low-level parts have their own detailed unit tests).