The iOS test harness only includes the current test directory in its
app bundles, but the tests need access to all source code.

Change-Id: I8a902b183bc2745b4fbfffef867002d573abb1f5
Reviewed-on: https://go-review.googlesource.com/37961
Run-TryBot: Elias Naur <elias.naur@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>