adding shellcheck in ci/cd

.github/workflows/testing.yml

@@ -22,6 +22,7 @@ jobs:
         pip install -r tests-requirements.txt
         pip install -r requirements.txt
         npm install -g remark-cli remark-validate-links
+        sudo apt-get install shellcheck
     - name: Run with 1 worker
       run: |
         ./tests.sh