misc/cgo/test: skip Test9400 on Alpine
misc/cgo/test: skip Test9400 on Alpine

Alpine has a known issue where setgid clobbers the Go stack (#39857).
misc/cgo/test skips other tests that use setgid on Alpine, but not
this one. It's not clear to me why this test *used to* pass, but when
I refactored misc/cgo/test in CL 447355 it started failing.

Disable this test on Alpine, like the other setgid tests.

Change-Id: I2e646ef55e2201a4f0b377319d719a011ec847f0
Reviewed-on: https://go-review.googlesource.com/c/go/+/448355
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Run-TryBot: Austin Clements <7ea35d812706d9213868749011af1ed4fa2f6aa0@google.com>
Reviewed-by: Cherry Mui <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>

golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go