tests: Explicit usage of Python 3

Use the program search path to find the Python 3 interpreter.

Patch created mechanically by running:

  $ sed -i "s,^#\!/usr/bin/\(env\ \)\?python$,#\!/usr/bin/env python3," \
       $(git grep -l 'if __name__.*__main__')
Reported-by: NVladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Suggested-by: NDaniel P. Berrangé <berrange@redhat.com>
Suggested-by: NStefan Hajnoczi <stefanha@redhat.com>
Acked-by: NStefan Hajnoczi <stefanha@redhat.com>
Acked-by: NPaolo Bonzini <pbonzini@redhat.com>
Message-Id: <20200130163232.10446-5-philmd@redhat.com>
Signed-off-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>

tests/docker/travis.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 #
 # Travis YAML config parser
 #

tests/qapi-schema/test-qapi.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 #
 # QAPI parser test harness
 #

tests/vm/centos

-#!/usr/bin/env python
+#!/usr/bin/env python3
 #
 # CentOS image
 #

tests/vm/fedora

-#!/usr/bin/env python
+#!/usr/bin/env python3
 #
 # Fedora VM image
 #

tests/vm/freebsd

-#!/usr/bin/env python
+#!/usr/bin/env python3
 #
 # FreeBSD VM image
 #

tests/vm/netbsd

-#!/usr/bin/env python
+#!/usr/bin/env python3
 #
 # NetBSD VM image
 #

tests/vm/openbsd

-#!/usr/bin/env python
+#!/usr/bin/env python3
 #
 # OpenBSD VM image
 #

tests/vm/ubuntu.i386

-#!/usr/bin/env python
+#!/usr/bin/env python3
 #
 # Ubuntu i386 image
 #