tests: add test for the -socket option

Refactored webhook_test so that the test HTTP requests are made using an explicitly-provided http.Client, so we can run at least one test with the server bound to a socket instead of a port number, using an http.Client whose transport has been configured with a suitable Unix-domain or Windows named pipe dialer function.

testutils_windows.go

@@ -0,0 +1,22 @@
+//go:build windows
+// +build windows
+
+package main
+
+import (
+	"context"
+	"github.com/Microsoft/go-winio"
+	"net"
+	"net/http"
+)
+
+func prepareTestSocket(hookTmpl string) (socketPath string, transport *http.Transport, cleanup func(), err error) {
+	socketPath = "\\\\.\\pipe\\webhook-" + hookTmpl
+	transport = &http.Transport{
+		DialContext: func(ctx context.Context, _, _ string) (net.Conn, error) {
+			return winio.DialPipeContext(ctx, socketPath)
+		},
+	}
+
+	return socketPath, transport, nil, nil
+}