watchdog: tests and latest work

	modified:   avr_watchdog.c
	modified:   sim_interrupts.c
	modified:   sim_regbit.h
	new file:   ../../tests/atmega48_watchdog_register_test.c
	new file:   ../../tests/test_atmega48_watchdog_register_test.c
	modified:   ../../tests/test_atmega48_watchdog_test.c