tests: drivers: dac: loopback: use internal loopback on the twr_ke18f

Enable internal DAC to ADC loopback on the NXP TWR-KE18F
board.

Increase the ADC sample time to 5 microseconds since there is no
buffer between the DAC and ADC.

Signed-off-by: Henrik Brix Andersen <henrik@brixandersen.dk>