Write tests for usb_gadget.rs

[robertosw]

The file usb_gadget.rs contains the code to write a specified gadget (UsbGadgetDescriptor) into the directory /sys/kernel/config/usb_gadget/ and binds this gadget to an UDC.
After this, the Raspberry Pi is recognized by an host.

Currently this is manually tested. Some parts of this process (like checking if the host detects this one device correctly) are impossible to test in rust on device side.
However, it should be possible to write tests for some generic instance of the UsbGadgetDescriptor, which test if the values are written to data correctly