Move it to a _test.go file instead.

After CL 43190, the pair function is no longer used by the hpack
package, so it can be removed from it. It's still used by tests.

This can help avoid future readers spending time on figuring out
why pair exists in package, but is not used. It can also reduce
chance of the pair function unintentionally being used in the
package (if that needs to happen explicitly, then pair can be
easily moved back to package).

Change-Id: I12ee9b46f57a522bd09e5d0ee22296df4dee1f35
Reviewed-on: https://go-review.googlesource.com/43851Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>