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/init/init_directory_structure.sh-rootless.yaml

@@ -19,7 +19,7 @@ tests:
             #!/usr/bin/env bash
             set -eu
 
-            gpg --batch --import /raw/private.asc
+            gpg --batch --import "$TMP_RAW_GPG_KEY"
   - it: skips gpg script block for disabled signing
     asserts:
       - equal: