chore: eliminate registerTests cmake module (from project-template)

This module is used in this one instance only, hence there is no justification to make it part of the project-template. The module is a lot more complicated than necessary (partialy due to the flawed attempt to make it generic). Simplifiction and cleanup can be done in a separate step.

Note: The project does not compile on Ubuntu 20.04, but it does not do so even without these modifications!