cpp.gitlab-ci.yml: add a testing stage

The build stage only build everything, then we have a testing stage.

it seems to be working: https://gitlab.com/tango-controls/hdbpp/libhdbpp-timescale/-/pipelines/2104555322

I'd like to have a variable to deactivate the stage by default as most device servers do not have tests, but I couldn't figure it out yet…