tests: Revise LDAP config + setup (#3514)

* chore: Use  white-space in query filters to improve readability

* tests: LDAP ENV query filters documented

- These filters remain roughly the same as they were before. The conditions are the same, but restructured to make the complimentary constraints more separated from the actual target attribtues.
- The DOMAIN query additionally includes the `mailAlias` from `PostfixBookMailAccount` class as well.
- Heavy inline documentation breaking down how the filters work for any maintainer to reference. This will likely be migrated after revision into our user docs for LDAP. Some quirks have also been noted with advice for future changes.

* tests: LDAP - Support test-case specific containers

A bit more complicated than other test files due to the larger ENV config array that most containers may need to share.

Example introduced with the test-case checking custom config file support.

* tests: Adjust LDAP test configs

- Paths for `.ldif` files used with volumes shortened
- Postfix LDAP `.cf` files adjusted to conventions used in LDAP tests.

test/config/ldap/overrides/ldap-groups.cf

@@ -1,10 +1,10 @@
 # Testconfig for ldap integration
-bind                     = yes
-bind_dn                  = cn=admin,dc=domain,dc=com
-bind_pw                  = admin
-query_filter             = (&(mailGroupMember=%s)(mailEnabled=TRUE))
-result_attribute         = mail
-search_base              = ou=people,dc=domain,dc=com
-server_host              = mail.domain.com
-start_tls                = no
-version                  = 3
+bind             = yes
+bind_dn          = cn=admin,dc=example,dc=test
+bind_pw          = admin
+query_filter     = (&(mailGroupMember=%s)(mailEnabled=TRUE))
+result_attribute = mail
+search_base      = ou=users,dc=example,dc=test
+server_host      = mail.example.test
+start_tls        = no
+version          = 3