The test used to import ../callback.  I forget why that ever worked,
but it probably had something to do with the shared libraries we used
to use with SWIG.  It doesn't work today.

Change-Id: Ib83d6c398aa46bf2fc66320b47b6e6d9897ee0b7
Reviewed-on: https://go-review.googlesource.com/7004Reviewed-by: David Crawshaw <crawshaw@golang.org>