time: add a regression test for time.AfterFunc goroutine starvation

time: add a regression test for time.AfterFunc goroutine starvation

The test is skipped on wasm platforms for now, because it
successfully detects a starvation bug on those platforms.

For #65178.

Change-Id: I05d28f1c7be99fcab67ec4dfaa38f412e11fd3cb
Reviewed-on: https://go-review.googlesource.com/c/go/+/557038
Auto-Submit: Bryan Mills <bcmills@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>