tests: be explicit about using 'python3' instead of 'python'

This fixes running tests in virtualenvs (or on distros) that no longer
have a symlink from python to python2 or python3.

Closes #7602

tests/dictserver.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 # -*- coding: utf-8 -*-
 #***************************************************************************
 #                                  _   _ ____  _

tests/negtelnetserver.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 # -*- coding: utf-8 -*-
 #
 #  Project                     ___| | | |  _ \| |

tests/smbserver.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 # -*- coding: utf-8 -*-
 #
 #  Project                     ___| | | |  _ \| |

tests/util.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 # -*- coding: utf-8 -*-
 #
 #  Project                     ___| | | |  _ \| |