test: use official unittest plugin (#410)

### Description of the change

Switch to official `helm-unittest` which is now alive again at https://github.com/helm-unittest/helm-unittest

### Applicable issues

- ref #199

### Checklist

<!-- [Place an '[X]' (no spaces) in all applicable fields. Please remove unrelated fields.] -->

- [ ] 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`

Reviewed-on: https://gitea.com/gitea/helm-chart/pulls/410
Reviewed-by: justusbunsi <justusbunsi@noreply.gitea.io>
Reviewed-by: pat-s <pat-s@noreply.gitea.io>
Co-authored-by: Michael Kriese <michael.kriese@visualon.de>
Co-committed-by: Michael Kriese <michael.kriese@visualon.de>

Makefile

@@ -9,4 +9,4 @@ readme: prepare-environment
 
 .PHONY: unittests
 unittests:
-	helm unittest --helm3 --strict -f 'unittests/**/*.yaml' ./
+	helm unittest --strict -f 'unittests/**/*.yaml' ./