.gitlab-ci.yml: Use same stage names for independent jobs

According to the documentation <https://docs.gitlab.com/ce/ci/yaml/#stages>:
“Jobs of the same stage are run in parallel”.

Therefore a stage test_unit seems to make sense where all unit tests would be
launched in parallel.

.gitlab-ci.yml

@@ -5,10 +5,10 @@ before_script:
   - make germline
 
 stages:
-  - test_browser-unit
+  - test_unit
 
 test_browser_unit:
-  stage: test_browser_unit
+  stage: test_unit
   script: make unit_browser