util/testutil: add RandomUnprivilegedPort function

So we can randomize ports used for listening in tests, so they can run
in parallel.

Signed-off-by: Mateusz Gozdek <mgozdekof@gmail.com>

util/testutil/port.go

@@ -0,0 +1,35 @@
+// Copyright 2021 The Prometheus Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package testutil
+
+import (
+	"net"
+	"testing"
+)
+
+// RandomUnprivilegedPort returns valid unprivileged random port number which can be used for testing.
+func RandomUnprivilegedPort(t *testing.T) int {
+	t.Helper()
+
+	listener, err := net.Listen("tcp", ":0")
+	if err != nil {
+		t.Fatalf("Listening on random port: %v", err)
+	}
+
+	if err := listener.Close(); err != nil {
+		t.Fatalf("Closing listener: %v", err)
+	}
+
+	return listener.Addr().(*net.TCPAddr).Port
+}