Fix destroying UI in some tests.

These tests were creating two UI instances in the same container.  This
caused the second one to replace the first without destroying the first.
This caused test contamination since there could be extra CastProxy
instances.  This wouldn't always happen since the no-longer-referenced
objects would be garbage-collected eventually; but depending on timing,
the CastSender tests might fail.

Change-Id: I7f4a008f56224ee33bee15c5e0370a225a5fc033

Author: TheModMaker

test/ui/ui_customization_unit.js

@@ -57,18 +57,22 @@ describe('UI Customization', () => {
     UiUtils.confirmElementMissing(container, 'shaka-caption-button');
   });
 
-  it('seek bar only created when configured', () => {
-    UiUtils.createUIThroughAPI(container, video, {addSeekBar: false});
+  it('seek bar only created when configured', async () => {
+    const ui =
+        UiUtils.createUIThroughAPI(container, video, {addSeekBar: false});
     UiUtils.confirmElementMissing(container, 'shaka-seek-bar');
+    await ui.destroy();
 
     UiUtils.createUIThroughAPI(container, video, {addSeekBar: true});
     UiUtils.confirmElementFound(container, 'shaka-seek-bar');
   });
 
-  it('big play button only created when configured', () => {
-    UiUtils.createUIThroughAPI(container, video, {addBigPlayButton: false});
+  it('big play button only created when configured', async () => {
+    const ui =
+        UiUtils.createUIThroughAPI(container, video, {addBigPlayButton: false});
     UiUtils.confirmElementMissing(container, 'shaka-play-button-container');
     UiUtils.confirmElementMissing(container, 'shaka-play-button');
+    await ui.destroy();
 
     UiUtils.createUIThroughAPI(container, video, {addBigPlayButton: true});
     UiUtils.confirmElementFound(container, 'shaka-play-button-container');