We've got away with not copying the testdata directories for the
standard library because the exec wrapper also pushes almost the
entire $GOROOT tree to the device, including testdata directories.

Similar to what the iOS exec wrapper does.

Change-Id: I91ef63ef84a658fc8843002890132c64b7c1d20e
Reviewed-on: https://go-review.googlesource.com/c/163626
Run-TryBot: Elias Naur <mail@eliasnaur.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>