strings: more thorough Replacer tests.
strings: more thorough Replacer tests.

This verifies existing behavior. Some replacements are arguably wrong
(these are marked with TODO) but changing behavior is left for a
follow-up CL.

Also fix that BenchmarkGenericMatch wasn't actually matching anything.

R=rsc, eric.d.eisner
CC=bradfitz, golang-dev
http://codereview.appspot.com/6488110
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go