misc/cgo/testcshared: use adb exec-out instead of adb shell

Adb exec-out is like adb shell except non-flaky in non-interactive
settings. Don't ask why.

Change-Id: I7ac3c72912883d80bc787c1d0fc101db6bae9c52
Reviewed-on: https://go-review.googlesource.com/c/go/+/170952


Run-TryBot: Elias Naur <mail@eliasnaur.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>