Improve test suite organisation

[cvmiert]

The test scripts (e.g. https://github.com/usabilla/php-docker-template/blob/master/test-cli.sh) control test suites that are run for different tags. This happens based on custom markers.

However the number of different markers currently used across multiple test files is causing confusion. The way the tests are organised and run could benefit from a better structured and transparent approach.