dm: test: usb: rework keyboard test

Allow the unit test to pass full 8 byte scan code sequences to the USB
keyboard emulation driver and to parse multi-byte escape sequences.

The following features are not yet tested:

* LED status
* caps-lock
* num-lock
* numerical pad keys

The following features are not yet implemented by the USB keyboard
driver and therefore not tested:

* modifiers for non-alpha-numeric keys, e.g. <SHIFT><TAB> and <ALT><F4>
* some special keys, e.g. <PRINT>
* some modifiers, e.g. <ALT> and <META>
* alternative keyboard layouts

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>