LDAP "test connection" button should check DN additions

LDAP "test connection" button currently:

• checks the LDAP host exists
• checks that authentication works with the supplied connection user DN / password

It would be nice if it:

• verified the existence of the base DN
• verified the existence of the user DN
• verified the existence of the group DN

It would be even nicer if it:

• verified each of the user attribute mappings
• verified each of the group attribute mappings