sleepLong() is sleep(300) — the single tunable
wait used by router / location tests where a macrotask isn't enough
(e.g. history.back(), delayed navigation, Navigate with replace,
useBeforeLeave). Tuning this one constant retimes every flaky wait.
Part of pota/use/test.
Takes no arguments.
Returns: Promise<void> that resolves after 300 milliseconds.