Based on changes from an earlier closed hostname PR from Oct 2021 with additional revision to use `assert_output` and more thorough checking of values expected in output.

This was originally a single test case in `tests.bats` intended for `supervisord` testing.

It seems at some point it got reassigned to a hostname override test container, and then migrated to separate test file for hostname override test by accident.

It now belongs in the correct place again, as hostname config should have nothing to do with a graceful shutdown?

Wait for SMTP port is left at the end to avoid additional start-up delays.

- I could do multiple container grep calls instead, but opted to match by lines in file. This better ensures values are being matched to the correct lines.
- Renamed the test case descriptions.
- Expanded test coverage of the 4th container as it represents another DNS config, while the 3rd is just the 4th container with the `SRS_DOMAINNAME` env added, no value in more coverage there.

These checks are performed in `mail_hostname.bats` with better coverage.

The original `fqdn-with-subdomain` is now `with-nis-domain` which is more accurate. A new test case will properly cover the default `--hostname` only config that is not a bare domain.

This commit just shifts the test cases, no new changes to any content beyond that.

- Fix a suggested change bug with quote wrapping an interpolated variable.
- Convert two other `_bash` methods that were missed from review.
- Apply the last two suggested changes from review.