tests: gadget0: delay between calls to trigger races

Attempt to be more brutal by delaying more often, instead of always
promptly servicing the usb stack.

This is implemented via using timer6 to do a known number of
microseconds busy delay, and so only works on platforms that have
reached at least core timer functionality, and provide the
rcc_apb1_frequency variable.

NOTE! This will _fail_ on devices using the st_usbfs drivers at present,
but the code _should_ work, and the tests land to verify that the
library fix, fixes the problem. (see subsequent commit)

tests/gadget-zero/Makefile.stm32f072disco

@@ -23,6 +23,7 @@ SHARED_DIR = ../shared
 
 CFILES = main-$(BOARD).c
 CFILES += usb-gadget0.c
+CFILES += delay.c
 
 VPATH += $(SHARED_DIR)