Bug 1343955 - part 0: Fix bug of browser_panel_keyboard_navigation.js r=smaug

browser_panel_keyboard_navigation.js sets "Tab" to first argument of
EventUtils.synthesizeKey().  However, this causes inputting "T", "a", "b" with
a key press.  The test intended to emulate pressing the "Tab" key.  So, it
should be "KEY_Tab".

Note that this will cause permanent orange with the following patches since
synthesizeKey() will detect this kind of mistake.

Differential Revision: https://phabricator.services.mozilla.com/D7412

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

browser/components/customizableui/test/browser_panel_keyboard_navigation.js

@@ -180,7 +180,7 @@ add_task(async function testTabKey() {
     let button = buttons[i];
     if (button.disabled)
       continue;
-    EventUtils.synthesizeKey("Tab", {shiftKey: true});
+    EventUtils.synthesizeKey("KEY_Tab", {shiftKey: true});
     Assert.equal(document.commandDispatcher.focusedElement, button,
       "The correct button should be focused after shift + tabbing");
   }