test: Fix uncaught errors while testing on Cast (#7345)

We get strange uncaught errors sometimes on Cast devices. These are
unreadable "script error" events that have nothing to do with our tests
(see https://sentry.io/answers/script-error/), and we intend to ignore
them.

However, our existing logic to ignore those is not sufficient, because
in addition to our own error handler (via
window.addEventListener("error")), Jasmine has its own unconditional
error handler (via window.onerror).

To take complete control over how these are handled, we need to remove
Jasmine's handler.

Jasmine's handler is installed at the top of its execute() function, so
our top-level beforeAll() is the best place to remove it.

test/test/boot.js

@@ -347,9 +347,13 @@ function configureJasmineEnvironment() {
     shaka.log.setLevel(shaka.log.Level.INFO);
   }
 
-  // Ensure node modules are loaded before any tests execute.
   beforeAll(async () => {
+    // Ensure node modules are loaded before any tests execute.
     await loadNodeModules();
+
+    // Replace jasmine's global error handler, since we have our own more
+    // nuanced version.
+    window.onerror = null;
   });
 
   const originalSetTimeout = window.setTimeout;