removed deps checks in unit tests

As Composer is now widely used in the PHP world, having to run composer
install before running the test suite is expected. This also has the
nice benefit of removing a bunch of code, making things easier to
maintain (there is only one place to declare a dev dependency), and
probably more.
merged branch fabpot/tests-simplification (PR #8796)

This PR was merged into the master branch.

Discussion
----------

removed deps checks in unit tests

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

As Composer is now widely used in the PHP world, having to run composer install before running the test suite is expected. This also has the nice benefit of removing a bunch of code, making things easier to maintain (there is only one place to declare a dev dependency), and probably more.

see fabpot/Silex#626 where we did the same a while ago for Silex.

Commits
-------

de50621 removed deps checks in unit tests