This also breaks fs_test into two parts
as the range tests test http's private httpRange
and I had to change the fs_test package from
"http" to "http_test" to use httptest which otherwise
has a cyclic depedency back on http.

Aside: we should start exposing the Range
stuff in the future.

R=rsc
CC=golang-dev
https://golang.org/cl/4261047