java/net/httpclient/http3/H3RequestRejectedTest.java fails intermittently (rarely) on macOS with javax.net.ssl.SSLHandshakeException: local endpoint (wildcard) and remote endpoint (loopback) ports conflict.

This is a well known issue on macOS, which can be avoided by having the client bind specifically to the loopback instead of the wildcard.

A couple of methods have been added to HttpServerAdapters (newClientBuilderForH3() / createClientBuilderForH3()) to obtain a client builder pre-configured to bind to the loopback.

Most HTTP/3 test call those methods, but java/net/httpclient/http3/H3RequestRejectedTest.java doesn't, hence the intermittent failure.