runtime/pprof: add block profile test Fixes issue 5993.
runtime/pprof: add block profile test
Fixes issue 5993.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/12665046

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang