Bug 1751010: Update JumpListBuilder tests for interface changes. r=mhowell

I couldn't see any value in running these for private and non-private mode, since all of those differences are up in Javascript land.

Differential Revision: https://phabricator.services.mozilla.com/D139608

widget/tests/unit/test_taskbar_jumplistitems.js

@@ -242,7 +242,10 @@ async function test_jumplist() {
     Ci.nsIWinTaskbar
   );
 
-  var builder = taskbar.createJumpListBuilder();
+  // Since we're only testing the general functionality of the JumpListBuilder
+  // et. al, we can just test the non-private browsing version.
+  // (The only difference between the two at this level is the App User Model ID.)
+  var builder = taskbar.createJumpListBuilder(false);
 
   Assert.notEqual(builder, null);