Make sure port is actually free and show what port was used.

internals/helpers/port.go

@@ -0,0 +1,20 @@
+package helpers
+
+import (
+	"fmt"
+	"net"
+)
+
+func IsFreePort(port int) bool {
+	addr, err := net.ResolveTCPAddr("tcp", fmt.Sprintf("localhost:%d", port))
+	if err != nil {
+		return false
+	}
+
+	l, err := net.ListenTCP("tcp", addr)
+	if err != nil {
+		return false
+	}
+	defer l.Close()
+	return true
+}