Improve LDAP test infrastructure.

The changes in this pull provide a reasonable means to stand up an AD and OpenLDAP servers that conform to a reasonable standard and execute LDAP tests against them. Tests requiring external fixtures do not execute by default; developers must explicitly execute them with -DenableLdapTests Maven flag.

The value of this new infrastructure is that it provides a test bed for verifying Spring configuration that we intend to support; the various authentication handler configurations are a notable example.