cmd/go: every test imports regexp
cmd/go: every test imports regexp

This fixes the bug Rob ran into when editing package bytes.
Regexp imports regexp/syntax, which imports bytes, and
regexp/syntax was not being properly recompiled during a
test of a change to package bytes.

R=golang-dev, r
CC=golang-dev
http://codereview.appspot.com/5555065

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