testing_hooks_util.ts: fix lint error

Author: dconeybe

packages/firestore/test/integration/util/testing_hooks_util.ts

@@ -30,7 +30,7 @@ export async function captureExistenceFilterMismatches(
   const results: ExistenceFilterMismatchInfo[] = [];
   const onExistenceFilterMismatchCallback = (
     info: ExistenceFilterMismatchInfo
-  ) => {
+  ): void => {
     _logWarn('zzyzx onExistenceFilterMismatchCallback', info);
     results.push(info);
   };