Upgrade phpunit to 7.5.x

Right now we are running phpunit 6.5.x (MDL-61678). It's time to bump to newer version.

Candidates are, right now: (source link):

• 7.x : supports php 7.1, 7.2 and 7.3. Reasonable changes required. Fully supported till Feb 2020.
• 8.x: supports php 7.2, 7.3 and 7.4. Huge changes required. Fully supported till Feb 2021.

Because of a better alignment with Moodle 3.7 PHP supported versions (see ongoing work @ MDL-63420) and changes being reasonable, this issue is about to upgrade to phpunit 7.5.

SUMMARY:

1. Upgrade composer to the target phpunit version (link to procedure).
2. Fullfill phpunit API required changes.
3. Fix other incompatibilities detected.
4. Verify that the new code is passing against ALL php supported versions.
5. Verify that behat continues passing without problem
6. Consider creating the next "Upgrade phpunit to X.Y.z" issue to evaluate its impact.
7. Send the issue to peer-review, yay!

TODO (after rolled upstream):

1. Create a new section like this @ the development Moodle Docs, explaining the main differences and linking to complete information.