[ci] Use gitlab-runner mechanism to fetch submodules

.gitlab-ci.yml

@@ -7,8 +7,6 @@ include:
   - project: biosignal/software/dev-profile
     file: /gitlab/python.yml
 
-tests:
-  before_script:
-    - git submodule sync
-    - git submodule update --init
-    - !reference [.snippets, test-prepare]
+variables:
+  GIT_SUBMODULE_STRATEGY: normal
+  GIT_SUBMODULE_DEPTH: 1