Introduce bash scripting unittests (#724)

### Description of the change

This adds an environment for unit testing our bash scripts, using [BATS](https://github.com/bats-core/bats-core).
It implements first tests for `config_environment.sh`.

### Benefits

Writing unit tests for bash scripts documents the expected behavior and allows it being a quality gate in our CI.

### Possible drawbacks

Not everyone is familiar with this approach and unit testing framework. Me neither, it took me some hours to get into it.

### Applicable issues

- Related to #691 where an issue in `config_environment.sh` was detected. It doesn't fixes it yet. This will be a dedicated Pull Request.

### Additional information

I've verified that the changes for Renovate are indeed working.

You may wonder why there is only one `run $PROJECT_ROOT/scripts/init-containers/config/config_environment.sh` and many `run execute_test_script` calls.
Usually, testing a script itself would be executing `run $PROJECT_ROOT/scripts/init-containers/config/config_environment.sh`. You then can assert the exit code and other things. Since the `config_environment.sh` exports environment variables and we are not able to access them from outside a `run` execution, the function `execute_test_script` wraps our script execution between environment comparison. Doing so allows us capture environment variables that were added/removed during script execution.

Reviewed-on: https://gitea.com/gitea/helm-chart/pulls/724
Reviewed-by: pat-s <pat-s@noreply.gitea.com>
Co-authored-by: justusbunsi <sk.bunsenbrenner@gmail.com>
Co-committed-by: justusbunsi <sk.bunsenbrenner@gmail.com>

.vscode/extensions.json

@@ -3,6 +3,7 @@
         "yzhang.markdown-all-in-one",
         "DavidAnson.vscode-markdownlint",
         "Tim-Koehler.helm-intellisense",
-        "esbenp.prettier-vscode"
+        "esbenp.prettier-vscode",
+        "jetmartin.bats"
     ]
   }