test/py: Add usb gadget binding test

Add a specific usb gadget binding test which check that
binding a driver without compatible string is working as expected.

the command "bind /usb@1 usb_ether" should give the following "dm tree"
command output:

[...]
 usb           0  [   ]   usb_sandbox           |-- usb@1
 usb_hub       0  [   ]   usb_hub               |   |-- hub
 usb_emul      0  [   ]   usb_sandbox_hub       |   |   `-- hub-emul
 usb_emul      1  [   ]   usb_sandbox_flash     |   |       |-- flash-stick@0
 usb_emul      2  [   ]   usb_sandbox_flash     |   |       |-- flash-stick@1
 usb_emul      3  [   ]   usb_sandbox_flash     |   |       |-- flash-stick@2
 usb_emul      4  [   ]   usb_sandbox_keyb      |   |       `-- keyb@3
 eth           4  [   ]   usb_ether             |   `-- usb@1
[...]

Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>
Cc: Marek Vasut <marex@denx.de>
Cc: Herbert Poetzl <herbert@13thfloor.at>
Reviewed-by: Simon Glass <sjg@chromium.org>