Test API in workers

[marcoscaceres]

Apart from the WebIDL checks, we are currently missing tests for workers.

• [ ] make sure badge can be set in a worker (or at least resolves as expected).
• [ ] maybe do all the checks that are done in the regular web page also in the worker.

[marcoscaceres]

Guidance on writing web worker tests: https://web-platform-tests.org/writing-tests/testharness-api.html#web-workers