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>

.gitea/PULL_REQUEST_TEMPLATE.md

@@ -23,7 +23,7 @@
 ### Applicable issues
 
 <!-- Enter any applicable Issues here (You can reference an issue using #). Please remove this section if there is no referenced issue. -->
-  - fixes #
+- Fixes #
 
 ### Additional information
 
@@ -39,5 +39,6 @@
 
 - [ ] Parameters are documented in the `values.yaml` and added to the `README.md` using [readme-generator-for-helm](https://github.com/bitnami-labs/readme-generator-for-helm)
 - [ ] Breaking changes are documented in the `README.md`
-- [ ] Templating unittests are added
+- [ ] Helm templating unittests are added (required when changing anything in `templates` folder)
+- [ ] Bash unittests are added (required when changing anything in `scripts` folder)
 - [ ] All added template resources MUST render a namespace in metadata