test: Add tests for serial numbers on tools

These tests make sure that any tablets with the capability to report a tool's
serial number do so properly, that the tool changes when another tool of the
same type with a different serial number is used, and that libinput doesn't
change the current tool when -1 is reported as the serial number (-1 is used for
special purposes by the linuxwacom driver).

Signed-off-by: Stephen Chandler Paul <thatslyude@gmail.com>