fix: ensure infinite effect loops are cleared after flushing

[Rich-Harris]

In #16746 we broke some behaviour around infinite effect loops, but due to a quirk of our test suite we didn't catch it.

This PR does two things — it augments the effect-loop-infinite test to ensure that the loop is cleared, and also adds a flushSync after the test logic runs, to ensure that nothing is 'left over' afterwards. This is also important for #16724, for reasons I don't yet totally grok.

It does not yet fix the issue, hence draft status

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