Bug 1267443 - [devtools] Wait for test function to be imported into browser toolbox. r=jdescottes

We weren't waiting for assertion function to be evaluated in the console.
This can later lead to ok/is functions being undefined when calling Toolbox.spawn.

Differential Revision: https://phabricator.services.mozilla.com/D94279

devtools/client/framework/browser-toolbox/test/helpers-browser-toolbox.js

@@ -96,7 +96,7 @@ async function initBrowserToolboxTask({
     );
   }
 
-  importFunctions({
+  await importFunctions({
     info: msg => dump(msg + "\n"),
     is: (a, b, description) => {
       let msg =