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.)