Increase testability of shell scripts (#752)

This is a preparation for #724 (bash script testing) which would be too complex to review if done there.

Reviewed-on: https://gitea.com/gitea/helm-chart/pulls/752
Co-authored-by: justusbunsi <sk.bunsenbrenner@gmail.com>
Co-committed-by: justusbunsi <sk.bunsenbrenner@gmail.com>

unittests/helm/deployment/signing-enabled.yaml

@@ -28,6 +28,8 @@ tests:
           value:
             - name: GNUPGHOME
               value: /data/git/.gnupg
+            - name: TMP_RAW_GPG_KEY
+              value: /raw/private.asc
       - equal:
           path: spec.template.spec.initContainers[2].volumeMounts
           value: