Correct type info in tests

In many places in the tests, we used "Object" or "*" or just no type
at all for various fakes.  These were all flagged by the new Closure
Compiler version we are adopting.

In some other places, we mixed up similar types or had the wrong
nullability on a type.

In still others, types were missing fields.

These issues were caught by a compiler upgrade.

Issue #2528

Change-Id: I324e0b28f7e30a4102aa26ec2c9901fa9732211b

test/ui/ui_unit.js

@@ -11,12 +11,12 @@ describe('UI', () => {
 
   /** @type {shaka.Player} */
   let player;
-  /** @type {!Element} */
+  /** @type {!HTMLLinkElement} */
   let cssLink;
 
   beforeAll(async () => {
     // Add css file
-    cssLink = document.createElement('link');
+    cssLink = /** @type {!HTMLLinkElement} */(document.createElement('link'));
     await UiUtils.setupCSS(cssLink);
   });