test_runner: gate event-loop drain and on_exit to node:test, drop BUN_TEST_DRAIN_EVENT_LOOP - #35401

The `unhandled_error_counter > prev_unhandled` check at line 3075 runs *after* `on_exit()` has already dispatched `process.on('exit')` handlers, so a drain-surfaced throw makes handlers see `code === 0` even though the process will exit 1. Node's `common.mustCall()` checker starts with `if (exitCode !== 0) return;`, so this prints spurious 'Mismatched' noise on top of the real error. Move the check inside the closure, between `handle_rejected_promises()` and `(*vm_ptr).on_exit()` (the post-closu

The `setTimeout(..., 500)` in the drain-throw fixture gives this test a hard 500ms wall-clock floor — the spawned `bun test` must spin `on_before_exit()` until the timer fires before it can exit. The 500ms was carried over verbatim from the ad-hoc repro; since the fixture's only test is a synchronous no-op and timers don't fire during collection/`tick_immediate_tasks`, a much smaller delay (~20ms — the reviewer's own analysis used 10ms) exercises the identical end-of-run drain path. REVIEW.md: '