main_test.go: Add to detect goroutine leaks

Updates #330

main_test.go

+package websocket_test
+
+import (
+	"fmt"
+	"os"
+	"runtime"
+	"testing"
+)
+
+func TestMain(m *testing.M) {
+	code := m.Run()
+	if runtime.NumGoroutine() != 1 {
+		fmt.Fprintf(os.Stderr, "goroutine leak detected, expected 1 but got %d goroutines\n", runtime.NumGoroutine())
+		os.Exit(1)
+	}
+	os.Exit(code)
+}