tests: drivers: i2s_api: Fix initialization of the test

- replace device_get_binding() calls that were searching for no longer
  existing I2S device labels with proper DEVICE_DT_GET_OR_NULL() calls
- move initialization of I2S devices that was previously performed
  in the test cases that were executed as first (what is no longer
  valid after moving to the new ztest API) to functions that are called
  before running each test in a given test suite

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>