Add data-event-id="$xxx" attributes to timeline items for easy selecting in tests

Split out from https://github.com/vector-im/hydrogen-web/pull/653

Example test assertions: db6d3797d7/test/e2e-tests.js (L248-L252)

```js
// Make sure the $abc event on the page has "foobarbaz" text in it
assert.match(
  dom.document.querySelector(`[data-event-id="$abc"]`).outerHTML,
  new RegExp(`.*foobarbaz.*`)
);
```

src/platform/web/ui/session/room/timeline/BaseMessageView.js

@@ -40,14 +40,17 @@ export class BaseMessageView extends TemplateView {
         if (this._interactive) {
             children.push(t.button({className: "Timeline_messageOptions"}, "⋯"));
         }
-        const li = t.el(this._tagName, {className: {
+        const li = t.el(this._tagName, {
+            className: {
                 "Timeline_message": true,
                 own: vm.isOwn,
                 unsent: vm.isUnsent,
                 unverified: vm.isUnverified,
                 disabled: !this._interactive,
                 continuation: vm => vm.isContinuation,
-        }}, children);
+            },
+            'data-event-id': vm.eventId
+        }, children);
         // given that there can be many tiles, we don't add
         // unneeded DOM nodes in case of a continuation, and we add it
         // with a side-effect binding to not have to create sub views,