net/http: deflake TestServerKeepAlivesEnabled_h{1,2}

This test assumes that two successive TCP connections will use different
source ports. This does not appear to be a universally safe assumption.

Rewrite the test to use httptrace to detect connection reuse instead.

Fixes #46707

Change-Id: Iebfbdfdeb77a1e6663a0c654dc847cc270c5d54d
Reviewed-on: https://go-review.googlesource.com/c/go/+/360854


Trust: Damien Neil <dneil@google.com>
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>