misc/cgo/test: make TestSetgidStress cheaper

misc/cgo/test: make TestSetgidStress cheaper

TestSetgidStress spawns 1000 threads, which can be expensive on
some platforms or slow builders. Run with 50 threads in short
mode instead.

This makes the failure less reproducible even with buggy code. But
one can manually stress test it (e.g. when a flaky failure appear
on the builder).

Fixes #53641.

Change-Id: I33b5ea5ecaa8c7a56f59c16f9171657ee295db47
Reviewed-on: https://go-review.googlesource.com/c/go/+/415677
Reviewed-by: Austin Clements <austin@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Cherry Mui <cherryyz@google.com>