[TASK] Tests: Refactor and activate dbal tests

Our unit test environment is finally ready to execute existing dbal
unit tests, even if no database connection exists.

The patch refactors unit tests of ext:dbal, modernizes their codebase
and adds them to the build.

Resolves: #52527
Related: #51436
Releases: 6.2
Change-Id: I70507591dc89331d658d50196dfcaea002caa7d5