Add an example for testing a phoenix application with Gitlab CI

_Originally opened at !5074 by @nahtnam._

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## What does this MR do?

Add an example on how to setup testing on phoenix 

## Are there points in the code the reviewer needs to double check?

Grammar and formatting

## Why was this MR needed?

https://twitter.com/gitlab/status/749898132008337408

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- Tests
  - [ ] All builds are passing
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !6794