Test improvements

[Rich-Harris]

Couple of things that should follow #211 that I noticed while working on it:

• We shouldn't be using waitFor APIs, as they make tests inherently imprecise
• We need a way to test the SSR output, pre-hydration, and the post-hydration state of the page

[ehrencrona]

For the second point, what the sapper tests do is to call start manually. This was easy to override in client.js, but now start is instead called inside a script block rendered in framework code.

The simplest solution I could think of was to add something like preventStart to the render options, which would suppress the start (and probably also store start in a global variable so it can be called by the tests). Do you think it would be a viable solution?

[Rich-Harris]

Yeah, I was trying to think of a way that didn't feel like too much of a hack, but the simplest solution for now is something like that. Perhaps a startGlobal option in svelte.config.js that, if present, turns

start(...)

into

window.${options.startGlobal} = () => start(...)