tests: drivers: uart: mix_fifo_poll: Change fixture to uart_loopback

gpio_loopback fixture is used for single pair of shortened pins. For
this test 2 pairs are needed (cts-rts + tx-rx) so gpio_loopback does
not describe it correctly.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>