gitlab-ci: Don't upload test artifacts

There's no need to upload test artifacts since they don't produce anything
that we care about so far, while this phase slows down the pipeline execution.

!436
2 jobs for master in 4 minutes and 6 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #212727
build-mutter

00:01:28

 
  Test
passed #212728
test-mutter

00:02:38