tests(refactor): Extract mail account management tests from tests.bats (#3055)

* chore: Extract out accounts test cases from `tests.bats`

Standard test file format, the test cases have been copied over unmodified.

* chore: Revise test case descriptions

* tests(refactor): `accounts.bats`

Revised test cases:
- Some common test case logic extracted to test methods.
- Update direct user management commands to use the `setup email ...` variants.
- Improved assertions.
- Removed `sleep 2` lines as the need for that is ambiguous (may no longer be relevant?)
- Additional commentary for maintaining
- Two test cases for missing `postfix-accounts.cf` opted to just run the image without any volumes instead, as the `without-accounts/` folder  was empty anyway. 2nd test case can instead use a single `docker run` to check  the newly created`postfix-accounts.cf` content.
- `test/config/without-accounts/` remains as `open_dkim.bats` presently uses it.

* chore: Remove unnecessary account removal assert

Traced this back to the original PR where it appears to have been a typo and was probably intended as a cleanup on the `user4` account. Not necessary, removing.

* chore: Rename `accounts.bat` -> `account_management.bats`

---------

* feedback: Avoid `ls` for detecting directories

Replace `ls -d` approach from original test cases

Co-authored-by: Georg Lauterbach <44545919+georglauterbach@users.noreply.github.com>

* feedback: Remove asserting empty output on failure

Co-authored-by: Georg Lauterbach <44545919+georglauterbach@users.noreply.github.com>

test/helper/common.bash

@@ -353,6 +353,9 @@ function _add_mail_account_then_wait_until_ready() {
   local MAIL_PASS="${2:-password_not_relevant_to_test}"
   local CONTAINER_NAME=$(__handle_container_name "${3:-}")
 
+  # Required to detect a new account and create the maildir:
+  _wait_for_service changedetector "${CONTAINER_NAME}"
+
   _run_in_container setup email add "${MAIL_ACCOUNT}" "${MAIL_PASS}"
   assert_success