[FIX] web: legacy dialog: select only one o_dialog_container to append in

Before this commit, when a test had multiple o_dialog_container elements (the one where we put the
actual modal), a legacy dialog was appended multiple times in the DOM messing around with the test

After this commit, we force dialogs to append once by selecting the first container it finds

addons/web/static/src/legacy/js/core/dialog.js

@@ -83,7 +83,7 @@ var Dialog = Widget.extend({
             $content: false,
             buttons: [{text: _t("Ok"), close: true}],
             technical: true,
-            $parentNode: false || $(".o_dialog_container"),
+            $parentNode: false || $(document.body.querySelector(".o_dialog_container")),
             backdrop: 'static',
             renderHeader: true,
             renderFooter: true,