Fix intro comments in test configs

tests/data/slapd-dn.conf

-# stand-alone slapd config -- for testing (with refint overlay)
+# stand-alone slapd config -- for testing DNs
 # $OpenLDAP$
 ## This work is part of OpenLDAP Software <http://www.openldap.org/>.
 ##

tests/data/slapd-emptydn.conf

-# stand-alone slapd config -- for testing (with refint overlay)
+# stand-alone slapd config -- for testing empty DNs
 # $OpenLDAP$
 ## This work is part of OpenLDAP Software <http://www.openldap.org/>.
 ##

tests/data/slapd-proxycache.conf

-# proxy cache slapd config 
+# proxy cache slapd config -- for testing
 # $OpenLDAP$
 ## This work is part of OpenLDAP Software <http://www.openldap.org/>.
 ##

tests/data/slapd-valsort.conf

-# stand-alone slapd config -- for testing (with unique overlay)
+# stand-alone slapd config -- for testing (with valsort overlay)
 # $OpenLDAP$
 ## This work is part of OpenLDAP Software <http://www.openldap.org/>.
 ##