Avoid logspam: "Cannot find menu item with id ..."

The extension frequently tries to remove context menus that do not
exists, which results in errors like:

> Error: Cannot find menu item with id firefox-container-1

because starting from Firefox 136, the contextMenus.remove method
rejects if the menu item does not exist. To avoid logspam, catch it.

References:

- https://bugzilla.mozilla.org/show_bug.cgi?id=1688743
- https://developer.mozilla.org/en-US/docs/Mozilla/Firefox/Releases/136#changes_for_add-on_developers

test/common.js

@@ -32,6 +32,10 @@ const buildDom = async ({background = {}, popup = {}}) => {
         window.crypto = {
           getRandomValues: arr => crypto.randomBytes(arr.length),
         };
+        // By default, the mock contextMenus.remove() returns undefined;
+        // Let it return a Promise instead, so that .then() calls chained to
+        // it (in src/js/background/assignManager.js) do not fail.
+        window.browser.contextMenus.remove.resolves();
       }
     }
   };