chore: test for batches flush fix

[dummdidumm]

Test for #16912

Also some explanation what the bug was:

1. async batch kicks off
2. outer async work succeeds, still something pending, so doesn't do anything for now
3. something unrelated writes to a signal (in the remote functions case it's the query writing to loading, raw etc), which creates a new batch
4. new batch executes. since there are multiple batches it takes the previous value which means if block is still alive. commits that, since no async work from the perspective of this branch
5. inner async work succeeds. now the batch has zero pending async work so it can flush. But the if block is no longer dirty since it was done by the other batch already -> never undos the other work

#16912 fixes it by still traversing the tree which means the if block deletion is scheduled to commit later, which it then does

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pnpm add https://pkg.pr.new/svelte@16915