[TASK] runTests.sh: Proper validation of -a option

Plus: No variable fallback magick in docker-compose.yml,
this belongs to runTests.sh

Change-Id: I9ec8d1ecba695061155807fd44d0a3cb3f342cfd
Resolves: #93514
Releases: master, 10.4
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/67919


Tested-by: Simon Gilli <typo3@gilbertsoft.org>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Richard Haeser <richard@richardhaeser.com>
Reviewed-by: Simon Gilli <typo3@gilbertsoft.org>
Reviewed-by: Richard Haeser <richard@richardhaeser.com>