feat: test backup directory is writable to avoid chown

[smoldenhauer-ish]

No description provided.

[HoustonPutman]

Thanks for making the PR!

The tests do cover this use case, but I'm very glad you did manual testing to ensure that it worked for your use case!
Mind adding a short description of your testing to the PR description for posterity?

Also please add a changelog entry. You can find an example here: https://github.com/apache/solr-operator/pull/539/files#diff-5e33a9126c77070c5c0eb99dda42378496c379464576c543c5f9a731da91284f
Basically a short description (e.g. Avoid backup directory chown when directories are already writable), and links to the Github Issue and Github PR.

[smoldenhauer-ish]

We did some (manual) tests with the changed statement.
If the backup directory already contains several backups from ~7x400 collections the execution time of the init container cp-solr-xml dropped from >10minutes (executing the chown) down to 2-3 seconds (executing only the directory writable test)