tests: usb: fix build all test on qemu_cortex_m3

Reduce RAM disk size from 192 sectors down to 1 sector to solve linking
issue due to qemu_cortex_m3 target having too little RAM. The RAM disk
size does not really matter in this test case and should be as small as
possible.

Enable test random generator to solve missing sys_rand_get() required by
networking subsystem.

Signed-off-by: Tomasz Moń <tomasz.mon@nordicsemi.no>