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.go

@@ -0,0 +1,30 @@
+//go:build !windows
+// +build !windows
+
+package main
+
+import (
+	"context"
+	"io/ioutil"
+	"net"
+	"net/http"
+	"os"
+	"path"
+)
+
+func prepareTestSocket(_ string) (socketPath string, transport *http.Transport, cleanup func(), err error) {
+	tmp, err := ioutil.TempDir("", "webhook-socket-")
+	if err != nil {
+		return "", nil, nil, err
+	}
+	cleanup = func() { os.RemoveAll(tmp) }
+	socketPath = path.Join(tmp, "webhook.sock")
+	socketDialer := &net.Dialer{}
+	transport = &http.Transport{
+		DialContext: func(ctx context.Context, _, _ string) (net.Conn, error) {
+			return socketDialer.DialContext(ctx, "unix", socketPath)
+		},
+	}
+
+	return socketPath, transport, cleanup, nil
+}