[nrf fromtree] 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>
(cherry picked from commit 5dec22a)

Author: bjarki-andreasen

tests/drivers/spi/spi_loopback/src/spi.c

@@ -315,7 +315,13 @@ ZTEST(spi_loopback, test_spi_complete_multiple_timed)
 	 */
 	zassert_ok(pm_device_runtime_get(spec->bus));
 
-	/* since this is a test program, there shouldn't be much to interfere with measurement */
+	/*
+	 * since this is a test program, there shouldn't be much to interfere with measurement.
+	 * still let's wait for the console to complete printing so it does not complete and
+	 * suspend near the end of the spi transaction.
+	 */
+	k_msleep(10);
+
 	start_time = k_cycle_get_32();
 	spi_loopback_transceive(spec, &tx, &rx, 2);
 	end_time = k_cycle_get_32();