[gitlab] Add shell script unit testing with bats

.gitlab-ci.yml

@@ -24,3 +24,11 @@ lint:
     - find . -type f -name '*.sh' | xargs shfmt -d -i 2
     - echo "--------- CHECKING ZSH SHELLSCRIPTS -------------"
     - find . -type f -name '*.zsh' | xargs shfmt -d -i 2
+
+test:
+  stage: test
+  image: bats/bats:latest
+  before_script:
+    - bats --version
+  script:
+    - bats -r .