fix: Workaround postconf write settling logic (#2998)

* fix: Workaround `postconf` write settle logic

After updating `main.cf`, to avoid an enforced delay from reading the config by postfix tools, we can ensure the modified time is at least 2 seconds in the past as a workaround. This should be ok with our usage AFAIK.

Shaves off 2+ seconds roughly off each container startup, reduces roughly 2+ minutes off tests.

* chore: Only modify `mtime` if less than 2 seconds ago

- Slight improvement by avoiding unnecessary writes with a conditional check on the util method.
- Can more comfortably call this during `postfix reload` in the change detection cycle now.
- Identified other tests that'd benefit from this, created a helper method to call instead of copy/paste.
- The `setup email restrict` command also did a modification and reload. Added util method here too.

* tests(fix): `mail_smtponly.bats` should wait for Postfix

- `postfix reload` fails if the service is not ready yet.
- `service postfix reload` and `/etc/init.d/postfix reload` presumably wait until it is ready? (as these work regardless)

* chore: Review feedback - Move reload method into utilities

target/scripts/check-for-changes.sh

@@ -59,7 +59,7 @@ function _check_for_changes
     _log_with_date 'debug' 'Reloading services due to detected changes'
 
     [[ ${ENABLE_AMAVIS} -eq 1 ]] && _reload_amavis
-    postfix reload
+    _reload_postfix
     [[ ${SMTP_ONLY} -ne 1 ]] && dovecot reload
 
     _remove_lock