Add bats unit testing to repository

Will recursively look for `*.bats` files and run them with bats-core test suite. Simple sample unit tests for existing scripts are included in `.config/shell/rc.d/test`.

.gitlab-ci.yml

@@ -24,3 +24,13 @@ 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: alpine
+  before_script:
+    - apk add git bash
+    - git clone https://github.com/bats-core/bats-core.git /bats
+    - /bats/bin/bats --version
+  script:
+    - /bats/bin/bats -r .