tests/py: Fix test script for Python3 tempfile

tests/py: Fix test script for Python3 tempfile

When instantiating a temporary file using tempfile's TemporaryFile()
constructor, the resulting object's 'name' attribute is of type int.
This in turn makes print_msg() puke while trying to concatenate string
and int using '+' operator.

Fix this by using format strings consequently, thereby cleaning up code
a bit.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Acked-by: Pablo Neira Ayuso <pablo@netfilter.org>