[Composer] Add PHPUnit as deps

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | ~
| License       | MIT
| Doc PR        | TODO

This PR adds PHPUnit as dev dependencies for all packages. I have fixed it as stable not sure if it is the right choice.

I know fabpot considers PHPUnit as an external tool regarding #5748 but all tests are designed around it and cannot work without it, so, IMO it is really a dev dependency. Additionally, it will simplify new comers to execute the test suite regarding the DX initiative.
`~3.7` is equivalent to `>=3.7,<4.0` whereas the test suite works with PHPUnit `4.*` as you can see on travis.