runtime: allocate fewer bytes during TestEINTR

This will hopefully address the occasional "runtime: out of memory"
failures observed on the openbsd-arm-jsing builder:
https://build.golang.org/log/c296d866e5d99ba401b18c1a2ff3e4d480e5238c

Also make the "spin" and "winch" loops concurrent instead of
sequential to cut down the test's running time.

Finally, change Block to coordinate by closing stdin instead of
sending SIGINT. The SIGINT handler wasn't necessarily registered by
the time the signal was sent.

Updates #20400
Updates #39043

Change-Id: Ie12fc75b87e33847dc25a12edb4126db27492da6
Reviewed-on: https://go-review.googlesource.com/c/go/+/234538


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>