runtime: replace TestFutexsleep with TestTimediv

TestFutexsleep was originally created in CL 7876043 as a
regression test for buggy division logic in futexsleep. Several months
later CL 11575044 moved this logic to timediv (called by futexsleep).

This test calls runtime.Futexsleep, which temporarily disables
asynchronous preemption. Unfortunately, TestFutexSleep calls this from
multiple goroutines, creating a race condition that may result in
asynchronous preemption remaining disabled for the remainder of the
process lifetime.

We could fix this by moving the async preemption disable to the main
test function, however this test has had a history of flakiness. As an
alternative, this CL replaces the test wholesale with a new test for
timediv, covering the overflow logic without the difficulty of dealing
with futex.

Fixes #50749.

Change-Id: If9e1dac63ef1535adb49f9a9ffcaff99b9135895
Reviewed-on: https://go-review.googlesource.com/c/go/+/380058


Trust: Michael Pratt <mpratt@google.com>
Run-TryBot: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>