build: androidtest.bash has to use copy for files to adb-sync.

androidtest.bash copies some go source to the android device
where the tests are going to run. It's necessary because some
tests require files and resources to be present. The copy is
done through adb sync. The script hoped faking the directory
using symlinks to work, but it doesn't. (adb sync doesn't follow
the symlinks) We need proper copy.

Change-Id: If55abca4958f159859e58512b0045f23654167e3
Reviewed-on: https://go-review.googlesource.com/2827


Reviewed-by: David Crawshaw <crawshaw@golang.org>