[ci] decide build/test job based on selection job result

.gitlab-ci/default.yml

@@ -27,7 +27,7 @@ select tests:
       else
           echo "Received '$TRIGGER_SOURCE' as pipeline trigger event"
           echo "Skipping test selection, build/test stages will consider all tests!"
-          echo "{}" >> ../affectedtests.json
+          touch affectedtests.json
       fi
   artifacts:
     paths:
@@ -40,8 +40,8 @@ build dumux:
   script:
     - |
       pushd build-cmake
-        make clean && make
-        if [[ "$TRIGGER_SOURCE" == "merge_request_event" ]]; then
+        make clean && make all
+        if [ -s ../affectedtests.json ]; then
           python3 ../bin/testing/runselectedtests.py -c ../affectedtests.json -b
         else
           python3 ../bin/testing/runselectedtests.py --all -b
@@ -64,7 +64,7 @@ test dumux:
   script:
     - |
       pushd build-cmake
-        if [[ "$TRIGGER_SOURCE" == "merge_request_event" ]]; then
+        if [ -s ../affectedtests.json ]; then
           python3 ../bin/testing/runselectedtests.py -c ../affectedtests.json -t
         else
           python3 ../bin/testing/runselectedtests.py --all -t