node tests: Clear message_user_ids.

This defends against future intra-test leaks.

frontend_tests/node_tests/message_store.js

@@ -90,6 +90,7 @@ function convert_recipients(people) {
 function test(label, f) {
     run_test(label, (override) => {
         message_store.clear_for_testing();
+        message_user_ids.clear_for_testing();
         f(override);
     });
 }

static/js/message_user_ids.js

@@ -13,6 +13,10 @@
 */
 const user_set = new Set();
 
+export function clear_for_testing() {
+    user_set.clear();
+}
+
 export function user_ids() {
     return Array.from(user_set);
 }