Add unit tests ensuring modal requests can start when conditional ones are running

[Firehed]

Generally:

async let autofillResult = await snapAuth.handleAutofill()
let modalResult = await snapAuth.startAuth(.handle("Test"))

I don't know enough about how to do UI testing to handle the modal situation, plus all of the network requests should ideally get mocked out. But this seems like a high-risk area for data races (even with Swift 6, in my limited testing; it's a logic problem rather than a synchronization one) and it will likely balloon in scope with #27.