Bug 1518005 - Fix intermittent devotools test. r=bgrins

Wait for devtools panel to be ready before attempting to use it.

Differential Revision: https://phabricator.services.mozilla.com/D27160

--HG--
extra : moz-landing-system : lando

devtools/client/inspector/test/browser_inspector_highlighter-07.js

@@ -29,8 +29,8 @@ function waitForPaused(dbg) {
   });
 }
 
-function createDebuggerContext(toolbox) {
-  const panel = toolbox.getPanel("jsdebugger");
+async function createDebuggerContext(toolbox) {
+  const panel = await toolbox.getPanelWhenReady("jsdebugger");
   const win = panel.panelWin;
   const { store, client, selectors, actions } = panel.getVarsForTests();
 
@@ -76,7 +76,7 @@ add_task(async function() {
 
   const target = await TargetFactory.forTab(gBrowser.selectedTab);
   await gDevTools.showToolbox(target, "jsdebugger");
-  const dbg = createDebuggerContext(toolbox);
+  const dbg = await createDebuggerContext(toolbox);
 
   await waitForPaused(dbg);