Add GitLab CI test job

This got lost on f/ci. We run the tests before the package build
so errors are more obvious and not hidden in package build logs.
2 jobs for ci in 1 minute and 14 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
passed #10370
librem5
builtin_tests

00:00:35

 
  Pkg
passed #10371
librem5
build-debian-package

00:00:38