Add CI tests for sample phoenix application

[fastjames]

In order to ensure the correctness of the example phoenix application, exercise it in the CI test suite.

[fastjames]

The test I added here should accomplish my original intent, by exposing the typespec mismatch (it does when I run mix dialyzer on the example app locally). However, it also finds a few other dialyzer errors too. I will see if I can knock those out as well in this PR (leaving them as separate commits in case they need to be split into multiple PRs).