avocado/utils/process.py: add transparent support for GDB

With current functionality being that it generates core dumps for
apps that receive such a signal.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
segfaulttest: introduce new test

That is useful to test the behaviour of an application that is run
inside the GNU debugger.

Signed-off-by: Cleber Rosa <crosa@redhat.com>