tests: drivers: uart: async_api: Align test to early RX buffer release

Modify the test to not rely on assumption that RX buffer is released
when it is fully released. RX buffer maybe released after any RX_RDY
event within the buffer, e.g. after each timeout driver may be
switching to the new buffer.

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