tests: Fix Python 3 detection on older GNU make versions

tests: Fix Python 3 detection on older GNU make versions

The $(SHELLSTATUS) variable requires GNU make >= 4.2, but Travis
seems to provide an older version.  Change the existing rules to
use command output instead of exit code, to make it compatible
with older GNU make versions.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>