(core) Adding testId to the widget iframe once it receives the ready message

Summary:
Iframe with custom widget is marked with a test class `test-custom-widget-ready` when
it receives the `ready` message from the rendered widget.

Test Plan: Added and updated. Existing test should pass.

Reviewers: georgegevoian

Reviewed By: georgegevoian

Subscribers: georgegevoian

Differential Revision: https://phab.getgrist.com/D4023

app/client/lib/domUtils.ts

@@ -0,0 +1,9 @@
+import {BindableValue, dom} from 'grainjs';
+
+/**
+ * Version of makeTestId that can be appended conditionally.
+ * TODO: update grainjs typings, as this is already supported there.
+ */
+export function makeTestId(prefix: string) {
+  return (id: string, obs?: BindableValue<boolean>) => dom.cls(prefix + id, obs ?? true);
+}