Resolve TS errors in tests

[geoffrich]

This PR resolves two TS errors that were reported on build. I wasn't able to find a better solution than casting to any, though I'm not a TypeScript expert.

The first issue is that Event.returnValue is a boolean and the test asserts that it is called with a string. However, per MDN, returnValue needs to be set to a string to prevent tab close in legacy browsers:

However note that not all browsers support this method, and some instead require the event handler to implement one of two legacy methods:

• assigning a string to the event's returnValue property

I couldn't figure out how to resolve this type conflict without potentially breaking the action for some browsers, so I cast to any.

The second issue is a mismatch between native DOM window and JSDOM window. I copied the offending lines from the main Svelte project which has a similar linting error in helpers.ts (see screenshot below). Wasn't sure how to solve this one either so I slapped an any on it for now.

[swyxio]

me neither! any is as good a solution as any :)