tests: drivers: i2s: Align nRF pinctrl with other gpio_loopback users

For nRF boards and lines that need to be connected with an external
loopback (SDOUT and SDIN), use the same pins as in other tests that
use the gpio_loopback fixture (like uart_async_api or regulator/fixed)
so that a common wiring can be used for all those cases.

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