parser testlib - use metaclass to mark all test functions keep_on_fail

This patch adds a python metaclass to wrap the test methods in the
subclasses of the template class AATestTemplate with the keep_on_fail
function, which sets the do_cleanup attribute to False when a testcase
failure occurs (i.e. an Exception is raised), and removes the manually
applied decorators to the caching tests that made use of this.

The downside to this approach is that the way metaclasses are declared
changed between python 2 and python 3 in an incompatible way. Since
python 3 is The Future™, I chose that approach and made the caching
and valgrind tests which use testlib be python3 (until this change,
they would have worked under either python 2 or python 3).

(An output message when a failure occurs is tweaked, to make the
output a little cleaner when verbose test output is requested and
failures occur.)

Signed-off-by: Steve Beattie <steve@nxnw.org>
Acked-by: Christian Boltz <apparmor@cboltz.de>

parser/tst/valgrind_simple.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 # ------------------------------------------------------------------
 #
 #   Copyright (C) 2013 Canonical Ltd.