[FIX] website_livechat: fix non deterministic reaction tour

Before this PR, the `test_complete_chatbot_flow_ui` would sometimes fail
when checking if the user can add a reaction to a message.

There are two issues:
- This step uses the `contains` helper and specify the live chat `ShadowRoot`
as the target. However, the `contains` helper uses a mutation observer
that observes the light DOM thus, changes in the shadow root are not
correctly observed.
- The target passed to the `contains` helper is the `ChatWindow` DOM
element but this element can be unmounted in-between.

This PR fixes this issue.

fixes runbot-56670

closes odoo/odoo#153382

Signed-off-by: Sébastien Theys (seb) <seb@odoo.com>

addons/web/static/tests/utils.js

@@ -327,7 +327,7 @@ class Contains {
                     this.def.reject(e); // prevents infinite loop in case of programming error
                 }
             });
-            this.observer.observe(document.body, {
+            this.observer.observe(this.options.target, {
                 attributes: true,
                 childList: true,
                 subtree: true,

addons/website_livechat/static/tests/tours/website_livechat_chatbot_flow.js

@@ -49,7 +49,7 @@ registry.category("web_tour.tours").add("website_livechat_chatbot_flow_tour", {
             // the thread has been persisted in the process
             async run() {
                 await contains(".o-mail-Message-actions [title='Add a Reaction']", {
-                    target: this.$anchor[0],
+                    target: this.$anchor[0].getRootNode(),
                     parent: [".o-mail-Message", { text: "I want to buy the software" }],
                 });
             },