compress/flate: fix test
compress/flate: fix test

The test was checking for a buffer to be empty but
actually racing with the background goroutine that
was emptying it.  Left a comment so that the check
is not reintroduced later.

Fixes issue 1557.

R=golang-dev, dsymonds
CC=golang-dev
http://codereview.appspot.com/4248063

wesolows/golang-old,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang,firebitsbr/golang-old,wesolows/golang,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang