tests: i2s: Update i2s_api test

1. Provide an option to loopback over separate I2S ports
2. Increase the buffer count in the loopback test to 4
3. Update test_i2s_transfer_rx_overrun test on how to
   handle RX overrun failures

Signed-off-by: Mahesh Mahadevan <mahesh.mahadevan@nxp.com>
6 files changed