tests: drivers: spi: loopback: wait for idle before timed test

Wait a bit before performing timed spi_loopback_transceive() in
test_spi_complete_multiple_timed to ensure the console is idle,
otherwise the console completing and suspending in the "background"
near the end of the spi transaction can introduce latency.

Signed-off-by: Bjarki Arge Andreasen <bjarki.andreasen@nordicsemi.no>