test: Add testlib.GetFreePort function

Some tests require picking ports, and today resort to hard-coding,
which is brittle.

This patch adds a testlib.GetFreePort function to help pick free ports.

It is not totally race-free, but is much better than hard-coding.

internal/testlib/testlib_test.go

@@ -76,3 +76,10 @@ func TestRewrite(t *testing.T) {
 		t.Errorf("basic rewrite failed")
 	}
 }
+
+func TestGetFreePort(t *testing.T) {
+	p := GetFreePort()
+	if p == "" {
+		t.Errorf("failed to get free port")
+	}
+}