All the remaining test cases but the last one were refactored here for a clean commit diff. The last test case will be refactored in the following commit.

Plenty of repeated logic spread across these test cases, now condensed into shared methods.

- As the majority of test cases are only running `open-dkim` helper, we don't actually have to wait for a full container setup. So an alternative container start is called.
- Also improves assertions a bit more instead of just counting lines.
- Some test cases don't bind mount all of `/tmp/docker-mailserver` contents, thus don't raise permission errors on subsequent test runs.
- Instead of `rm -f` on some config files, have opted to mount them read-only instead, or alternatively mount an anonymous empty volume instead.
- Collapsed the first three test cases into one, thus no `setup_file()` necessary.
- Shift the `_wait_for_finished_setup_in_container()` method into `_common_container_setup()` instead since nothing else is using `_common_container_start()` yet, this allows for avoiding the wait.

This makes these tests a bit more DRY, and enhances the raised quality issue with these tests. Now not only is the domain checked in the generated DNS dkim record, but we also verify the key size is corrected in the public and private keys via openssl.

- `__should_have_tables_trustedhosts_for_domain` shifted in each test case to just after generating the domains keys.
- Asserts `open-dkim` logs instead of just counting them.
- Added checks for domains that should not be present in a test case.
- Additional coverage and notes about the alias from vhost `@localdomain.com`
- Single assert statement with switch statement as all are using common args.

These cover the same test logic for the most part, the first domain could also be testing the custom selector.

`special_use_folders.bats` + `mailbox_format_dbox` can assert lines instead, removing the need for `--partial`.