build/androidtest.bash: script to run all tests in a repository on an
android device.

It assumes it runs in golang.org/x/mobile repository.

This script copies the repository source tree into the connected device
and invokes go test that runs using the binary compiled from
go_android_exec.go. go_android_exec arranges to push and run the test
binary to the right place in the android device.

Change-Id: I4ac24bcfd8e26e01cccd3d7c2dddaf6b3c14951e
Reviewed-on: https://go-review.googlesource.com/1680
Reviewed-by: David Crawshaw <crawshaw@golang.org>