fix(material-experimental/mdc-chips): expose avatar harness (#24499)

(cherry picked from commit 50efb92)

Author: andrewseguin

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

@@ -6,6 +6,7 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
+export * from './chip-avatar-harness';
 export * from './chip-harness';
 export * from './chip-harness-filters';
 export * from './chip-input-harness';