[Unit testing] Move internal bundle tests to the root tests folder

[netsensei]

Detailed description

Currently, all the tests are spread out over the various internal bundles (a tests folder per bundle). It would be better to move the test classes to the more general tests folder which lives in the root of the Symfony project.

Context

This change is important because:

• There's no need for tests in separated in bundles since we do not intend to release the core bundles as standalone packages.
• It would allow us to consolidate all the fixtures we use for testing (currently XML files all over the place)
• Less need for all kinds of bash scripts to run tests across folders.
• Improve simplicity towards contributors who are willing to write tests.

Possible implementation

• [x] Move tests to the tests folder but following the namespacing and directory structure of the bundles.
• [x] Consolidate the fixtures in a separate fixtures folder. Consider removing them all together if possible.
• [x] Configure phpunit.xml properly.
• [x] Refactor the bash scripts in the scripts folder
• [x] Refactor .travis.yml accordingly
• [x] Document how tests should be run

[netsensei]

We moved all tests to a seperate tests/ folder. But we still have work left with managing fixtures. These are currently still a part of the src bundles, not the tests bundles.