runtime: deflake TestPreemptionAfterSyscall

This test occasionally takes very slightly longer than the 3 second
timeout on slow builders (especially windows-386-2008), so increase
the timeout to 5 seconds. It fails with much longer timeouts on Plan
9, so skip it as flaky there.

Updates #41015.

Change-Id: I426a7adfae92c18a0f8a223dd92762b0b91565e1
Reviewed-on: https://go-review.googlesource.com/c/go/+/379214


Trust: Austin Clements <austin@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>