exclude end-to-end tests from running when calling mix text

[vanderhoop]

As a developer who would loves instant feedback from my test suite,
I would to exclude the end-to-end tests from running when invoking mix test,
so that I don't have to wait for the end-to-end tests to finish after trivial changes.

Acceptance Criteria:

• when a developer types mix test, the elixir unit tests are run, but the end-to-end tests are not

Mix Documentation

• https://hexdocs.pm/mix/Mix.Task.html
• http://www.phoenixframework.org/docs/mix-tasks
• https://elixirschool.com/lessons/basics/mix-tasks/