[Tests] Improve test suite coverage for Routing

The goal of this issue is to write some new tests or modify existing tests to improve coverage. There have been a series of routing bugs that may be related, and improved test coverage may have been able to prevent them. Multiple people can help work on this issue - just comment with what you are working on, and be sure to read through other people's comments.

To do:
- read through some of the issues below
- Write a test to help cover one of the problem areas

Past bug reports/issues:
- https://github.com/emberjs/ember.js/issues/19497
- https://github.com/emberjs/ember.js/issues/18577
- https://github.com/emberjs/ember.js/issues/17494
- https://github.com/emberjs/ember.js/issues/17118
- https://github.com/emberjs/ember.js/issues/16349
- https://github.com/emberjs/ember.js/issues/14875
- https://github.com/emberjs/ember.js/issues/14721
- https://github.com/emberjs/ember.js/issues/12169
- https://github.com/emberjs/ember.js/issues/11563