ldap_test: add filter tests

The filters for user and group searches hadn't been included in our LDAP
tests. Now they are.

The concrete test cases are somewhat contrived, but that shouldn't
matter too much. Also note that the example queries I've used are not
supported in AD: https://stackoverflow.com/a/10043452



Signed-off-by: Stephan Renatus <srenatus@chef.io>