tests/libqtest: Allow setting expected exit status

Add qtest_set_expected_status function to set expected exit status of
child process. By default expected exit status is 0.

Signed-off-by: Yury Kotov <yury-kotov@yandex-team.ru>
Message-Id: <20190903162246.18524-3-yury-kotov@yandex-team.ru>
Acked-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>