fix(material-experimental/mdc-list): export missing harness symbols (#24175)

(cherry picked from commit beeecd3)

Author: mmalerba

src/material-experimental/mdc-list/testing/public-api.ts

@@ -11,4 +11,4 @@ export * from './list-harness';
 export * from './list-harness-filters';
 export * from './nav-list-harness';
 export * from './selection-list-harness';
-export {MatListItemSection} from './list-item-harness-base';
+export {MatListItemSection, MatSubheaderHarness, MatListItemType} from './list-item-harness-base';