net/http: direct server logs to test output in tests

net/http: direct server logs to test output in tests

Set a logger in newClientServerTest that directs the server
log output to the testing.T's log, so log output gets properly
associated with the test that caused it.

Change-Id: I13686ca35c3e21adae16b2fc37ce36daea3df9d5
Reviewed-on: https://go-review.googlesource.com/c/go/+/452075
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>