`examples/tests/simplewarning.sh` calls a generic avocado command,
which gets added to the path by the test code. That generic avocado
command is `scripts/avocado`, from the source repository, which
contains the unversioned `/usr/bin/env python`.

Under some environments, such as Fedora >= 29, there may be no
unversioned Python binary.  Let's respect the UNITTEST_AVOCADO_CMD
environment variable, and add the the directory containting that
binary to the PATH.
Signed-off-by: NCleber Rosa <crosa@redhat.com>