Enable back redis sessions testing once MDL-60978 has landed

With MDL-60978 applied, redis session tests are back to pass, so we can revert the changes performed to both:

• moodle-ci-runner: https://github.com/moodlehq/moodle-ci-runner/commit/f36e8081c50dfa7342b9e91736c1febeeedebbd6
• moodle-docker: https://github.com/moodlehq/moodle-docker/pull/100

In order to get them back to run with PHP >= 7.2

Note that the 3rd place where we had made conditional redis session runs is travis, see MDL-64530 ... but that's already reverted by MDL-60978 itself, so no action there is needed.