misc/cgo/test: use fewer threads in TestSetgidStress in long mode

misc/cgo/test: use fewer threads in TestSetgidStress in long mode

TestSetgidStress originally spawns 1000 threads for stress testing.
It caused timeout on some builders so CL 415677 reduced to 50 in
short mode. But it still causes flaky timeouts in longtest
builders, so reduce the number of threads in long mode as well.

Should fix #53641.

Change-Id: I02f4ef8a143bb1faafe3d11ad223f36f5cc245c6
Reviewed-on: https://go-review.googlesource.com/c/go/+/419453
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>