test(js): stop asserting exact timer tick counts in flaky JS timer tests

[dbrattli]

Timer.Elapsed.Subscribe works failed on the windows-latest JavaScript leg of an unrelated PR (run) with Expected: 10 - Actual: 5. It is a wall-clock race, and so is its sibling Timer with AutoReset = true works.

The race

let t = new Timers.Timer(50.)
let disp = t.Elapsed.Subscribe(fun ev -> res := !res + 5)
t.Start()
do! Async.Sleep 125
disp.Dispose()
do! Async.Sleep 50
equal 10 !res          // exactly two 50ms ticks

The test asserts the handler ran exactly twice inside a 125ms sleep. The second tick is due at 100ms, so there are 25ms of slack. On a loaded CI runner — where Windows timer resolution is ~15.6ms and scheduling is coarse — that tick slips past the window, one tick arrives, and res is 5. The same job also reported the Main.js file itself hitting its 20s timeout, which is the signature of a runner that was simply slow at that moment.

Nothing about the count of 2 is part of the contract being tested; it is an artifact of the sleep being 2.5× the interval.

The fix

Assert the semantics each test is named for:

• the handler fires repeatedly (more than one tick — this is what AutoReset = true means, and what a live subscription means), and
• it stops firing once the timer is stopped / the subscription is disposed.

The counter is read after Stop()/Dispose(), so a tick cannot land between the read and the unsubscribe and make the second assertion flaky in turn.

This is the same tolerance the neighbouring Assigning an event to a variable works (#1863) already uses — 200ms sleep, at least two ticks (acc > 2 |> equal true) — which has been stable, so the new budget is a proven one in this file rather than a guess.

Timer with AutoReset = false works is deliberately left alone: it asserts exactly one tick, which is the invariant it exists to check (AutoReset off ⇒ fires once), not a timing artifact.

Testing

Full JavaScript suite: 3049 passing, 0 failed. The two rewritten tests take ~300ms each, up from ~175ms — negligible against the 20s per-file budget.

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