Touchpad is causing the touchscreen to be detected as a "non-touch" screen

Summary: touchscreen/nontouch-automated misinterpret devices that are not touchscreens/screens as "non-touch" screens

Steps:
1. Run plainbox touchscreen cases
2. Check results

Expected results:
If system has touchscreen:
(1) touchscreen/multitouch-automated should be run and return "true":
because requirement "xinput.device_class == 'XITouchClass' and xinput.touch_mode == 'direct'" is matched

(2) touchscreen/nontouch-automated should be skipped:
because requirement "xinput.device_class == 'XITouchClass' and xinput.touch_mode != 'direct'" is not matched

(3) touchscreen/multitouch-manual should be skipped:
because depended test case "touchscreen/nontouch-automated" is supposed to be skipped

Actual results:
On a Stella testing machine, I see 3 xinput devices with the class "XITouchClass":
(1) One of them (the touchscreen device)'s touch_mode is "direct", so touchscreen/multitouch-automated is run and passed
=> OK

(2) The other 2 (non touchscreen devices) has device_classes that are not "direct", therefore touchscreen/nontouch-automated is run as well, interpreting these 2 devices as "non-touchscreen panels"
=> Not OK

(3) Since touchscreen/nontouch-automated is run, the case that depends on it touchscreen/multitouch-manual is triggered and run as well, asking user to confirm in this test case if system is indeed a "non-touchscreen" system
=> Not OK

Additional information:
The 3 XITouchClass devices detected in xinput:
-----
type: device
max_touch: 2
touch_mode: dependent
device_class: XITouchClass
id: 2
name: Virtual core pointer
-----
type: device
max_touch: 10
touch_mode: direct
device_class: XITouchClass
id: 11
name: SYNA7508:00 06CB:12A4
-----
type: device
max_touch: 2
touch_mode: dependent
device_class: XITouchClass
id: 13
name: SynPS/2 Synaptics TouchPad
-----

Plainbox versions:
ii plainbox 0.23+ppa~ubuntu14.04.1 all toolkit for software and hardware integration testing

ii plainbox-glmark2-es2-meta 0.19-1 amd64 Meta-package to selectively install glmark2-es2

ii plainbox-insecure-policy 0.23+ppa~ubuntu14.04.1 all policykit policy required to use plainbox (insecure version)

ii plainbox-provider-certification-client 0.19-1 all Client Certification provider for Plainbox

ii plainbox-provider-checkbox 0.22~ppa1~ubuntu14.04.1 amd64 CheckBox provider for PlainBox

ii plainbox-provider-oem 0.23 all plainbox oem provider

ii plainbox-provider-oem-stella 0.23 all plainbox oem provider for stella

ii plainbox-provider-resource-generic 0.20+ppa~ubuntu14.04.1 amd64 CheckBox generic resource jobs provider

ii python3-plainbox 0.23+ppa~ubuntu14.04.1 all toolkit for software and hardware testing (python3 module)