net/http: improve logging in TestServerSetKeepAlivesEnabledClosesConns

- Log the actual addresses reported, in case that information is relevant.

- Keep going after the first error, so that we report more information
  about the idle connections after they have been used. (Was the first
  connection dropped completely, or did it later show up as idle?)

- Remove the third request at the end of the test. It had been
  assuming that the address for a new connection would always be
  different from the address for the just-closed connection; however,
  that assumption does not hold in general.

Removing the third request addresses one of the two failure modes seen
in #55195. It may help in investigating the other failure mode, but I
do not expect it to fix the failures entirely. (I suspect that the
other failure mode is a synchronization bug in returning the idle
connection from the first request.)

For #55195.

Change-Id: If9604ea68db0697268288ce9812dd57633e83fbd
Reviewed-on: https://go-review.googlesource.com/c/go/+/478515
Reviewed-by: Damien Neil <dneil@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>

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