This is the test code:
Catalog c = CatalogManager.catalog(CatalogFeatures.builder().with(CatalogFeatures.Feature.PREFER, "system").build(), RESOURCES + "same_sys_pub.xml");
        assertEquals("http://local/base/dtd/system.dtd", c.matchPublic("http://openjdk.java.net/xml/catalog/dtd/system.dtd"));

and following are the relevant entries in same_sys_pub.xml catalog file:
 <system systemId="http://openjdk.java.net/xml/catalog/dtd/system.dtd" uri="system.dtd"/>
    <public publicId="http://openjdk.java.net/xml/catalog/dtd/system.dtd" uri="public.dtd"/>