unicode/utf16: add explicit tests for IsSurrogate
unicode/utf16: add explicit tests for IsSurrogate

Update issue 6956

Add tests for IsSurrogate.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/42570043

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