Tests: move to separate dir, use Jasmine instead of Mocha

I tried to follow the suggestions from squidfunk#7350 (comment) as best I can.

The suggested `iframe-worker` template uses Karma, but I didn't use it because 1) it's a complicated tool I don't understand and 2) it says it's deprecated and isn't even accepting bugfixes in https://github.com/karma-runner/karma Instead, I followed https://www.innoq.com/en/blog/2020/04/ts-jasmine-karma/#executingtestsonnode.js to set up Jasmine with ts-node, since that's all I need for this application. If you need something like Karma or whatever replaced it, you can always rework this later.

OTOH, let me know if there's more I can do to make this easier for you to review/maintain. I'm trying to make the minimal changes to the project structure possible, so that it's easy to change anything about this setup later.