CI: fix test collisions between branches (again)

CI: fix test collisions between branches (again)

The build-netlab job was side-effecting the test-* jobs,
and if for some reason Gitlab scheduled build-netlab before
other pipeline's test-* jobs finished, these jobs got a wrong
binary, possibly failing. Solved by using explicit artifacts, which is
not the fastest way to do this (we could keep the binaries named there)
but it's the gitlab-right way to do this.

(Re-committed, was accidentally removed by one of previous commits)