Add missing getPluginRepositories method to TestFessConfig

This method returns a String representing plugin repository URLs
and is required by the FessConfig interface.

Author: claude

src/test/java/org/codelibs/fess/cors/DefaultCorsHandlerTest.java

@@ -875,6 +875,11 @@ public Integer getStorageMaxItemsInPageAsInteger() {
             return 100;
         }
 
+        @Override
+        public String getPluginRepositories() {
+            return "";
+        }
+
         @Override
         public String getPluginVersionFilter() {
             return null;