net/http: add support for running HTTP tests against HTTP/3

Add support within clientserver_test.go to bring up a test HTTP/3 server
and client when http3Mode testMode option is passed.

To be able to reuse net/http/httptest, net/http/httptest.Server.StartTLS
(and Start) have been modified so they can be called with a nil
Listener. In such cases, both methods will behave identically as usual,
but will not actually make its server serve or set its transport dialer,
both of which requires having a listener. This should be a no-op for
regular users of the package, whose entrypoint via functions such as
NewServer will automatically set a local listener.

Actually enabling HTTP/3 for our tests will be done in a separate CL.

For #70914

Change-Id: Ibc5fc83287b6a04b46e668a54924761a92b620a4
Reviewed-on: https://go-review.googlesource.com/c/go/+/740122
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Nicholas Husin <husin@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

0 files changed, 0 insertions, 0 deletions