net/http: quiet some log spam in tests

One of these tests creates a bunch of connections concurrently, then
discovers it doesn't need them all, which then makes the server log
that the client went away midway through the TLS handshake. Perhaps
the server should recognize that as a case not worthy of logging
about, but this is a safer way to eliminate the stderr spam during go
test for now.

The other test's client gives up on its connection and closes it,
similarly confusing the server.

Change-Id: I49ce442c9a63fc437e58ca79f044aa76e8c317b5
Reviewed-on: https://go-review.googlesource.com/c/go/+/179179


Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>