os/signal: split up sleeps waiting for signal

Try to deflake TestNohup.

The kernel will deliver a signal as a thread returns from a syscall.
If the only active thread is sleeping, and the system is busy,
the kernel may not get around to waking up a thread to catch the signal.
Try splitting up the sleep, to give the kernel another change to deliver.

I don't know if this will help, but it seems worth a try.

Fixes #33174

Change-Id: I34b3240af706501ab8538cb25c4846d1d30d7691
Reviewed-on: https://go-review.googlesource.com/c/go/+/194879


Reviewed-by: Bryan C. Mills <bcmills@google.com>