[nrf fromtree] tests: drivers: uart: async_api: Use custom long buffer for nrf54h20

nrf54h20dk_nrf54h20_cpurad by default has less RAM dedicated for
DMA transfers and default 1k buffer cannot be used for
uart_async_long_buf case. Use custom value.

Signed-off-by: Krzysztof Chruściński <[email protected]>
(cherry picked from commit d480093)

Author: nordic-krch

tests/drivers/uart/uart_async_api/boards/nrf54h20dk_nrf54h20_cpurad.conf

@@ -0,0 +1 @@
+CONFIG_TEST_LONG_BUFFER_SIZE=128