fix shebang lines in the tests to take advantage of any activated python environment

tests/test_dns.py

-#!/usr/bin/python3
+#!/usr/bin/env python3
 #
 # Tests the DNS configuration of a Mail-in-a-Box.
 #

tests/test_mail.py

-#!/usr/bin/python3
+#!/usr/bin/env python3
 # Tests sending and receiving mail by sending a test message to yourself.
 
 import sys, imaplib, smtplib, uuid, time

tests/test_smtp_server.py

-#!/usr/bin/python3
+#!/usr/bin/env python3
 import smtplib, sys
 
 if len(sys.argv) < 3: