When a test binary runs for too long, the go command sends it a
SIGQUIT to force a backtrace dump. On Android, the exec wrapper
will instead receive the signal and dump its backtrace.

Forward SIGQUIT signals from the wrapper to the wrapped process
to gain useful backtraces.

Inspired by issuse 25519; this CL would have revealed the hanging
test directly in the builder log.

Change-Id: Ic362d06940d261374343a1dc09366ef54edaa631
Reviewed-on: https://go-review.googlesource.com/114137
Run-TryBot: Elias Naur <elias.naur@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>