Add example projects to CI

Also run example projects during CI.

This way we can find problems with external usage.

.gitlab-ci.yml

 stages:
     - init
+    - test_projects
     - build_mpich_gcc8
     - build_openmpi_gcc8
     - build_mpich_gcc9
@@ -483,6 +484,36 @@ build-openmpi-complete-gcc10:
             - badges/
     allow_failure: false
 
+test-project-cmake:
+    stage: test_projects
+    image: gitlab.version.fz-juelich.de:5555/slms/loadbalancing/openmpi-gcc10-full:latest
+    tags:
+        - public-docker
+    when: always
+    script:
+        - ./ci/test_project_cmake.sh
+    allow_failure: false
+
+test-project-cmake-subdir:
+    stage: test_projects
+    image: gitlab.version.fz-juelich.de:5555/slms/loadbalancing/openmpi-gcc10-full:latest
+    tags:
+        - public-docker
+    when: always
+    script:
+        - ./ci/test_project_cmake_subdir.sh
+    allow_failure: false
+
+test-project-makefile:
+    stage: test_projects
+    image: gitlab.version.fz-juelich.de:5555/slms/loadbalancing/openmpi-gcc10-full:latest
+    tags:
+        - public-docker
+    when: always
+    script:
+        - ./ci/test_project_makefile.sh
+    allow_failure: false
+
 release:
     stage: release
     only: