net/http: split Trailers tests into two halves

net/http: split Trailers tests into two halves

The old test was in client_test.go but was a mix of four things:

- clients writing trailers
- servers reading trailers
- servers writing trailers
- clients reading trailers

It definitely wasn't just about clients.

This moves it into clientserver_test.go and separates it into two
halves:

- servers writing trailers + clients reading trailers
- clients writing trailers + servers reading trailers

Which still isn't ideal, but is much better, and easier to read.

Updates #13557

Change-Id: I8c3e58a1f974c1b10bb11ef9b588cfa0f73ff5d9
Reviewed-on: https://go-review.googlesource.com/17895
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Blake Mizerany <blake.mizerany@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>