tests: drivers: uart: uart_pm: Minor adjustments in the test

Modifying arbitrary delay to cover for receivers with timeout.
Adding a delay between polling and asynchronous checking to cover
for cases when CPU processing is fast enough so that both checks
may overlap.
Allow error code for uart_rx_disable call.

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