[TASK] Implement group "mysql" in functional tests

Add group "mysql" to select functional tests that require a MySQL
Server as a database backend due to testing functionality that is
only available on this platform.

Resolves: #79509
Releases: master
Change-Id: Ia4d653efc56673d666274dabdc86474ffafd4756
Reviewed-on: https://review.typo3.org/51438


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Tested-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Reviewed-by: Mona Muzaffar <mona.muzaffar@gmx.de>
Tested-by: Mona Muzaffar <mona.muzaffar@gmx.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>