net/http: allow multiple dials in TestTransportMaxConnsPerHost

If there is more than the expected single dial, the channel will block.
Allow at least one connection per client, and do the expected cleanup.

Updates #45570

Change-Id: Iaecd45298a7d7c591b7d7b1be13cea6e4a1e2e85
Reviewed-on: https://go-review.googlesource.com/c/go/+/310213


Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
Trust: Damien Neil <dneil@google.com>
Trust: Emmanuel Odeke <emmanuel@orijtech.com>