tests: drivers: uart: uart_async_api: fix error on test_write_abort

With the changes made in PR #76061, only on the F429ZI platform, in
the test_write_abort test,the number of received data (2) is not equal
to the number of sent data (1). It looks like uart_tx_abort is done very
quickly, and the callback doesn't have enough time to deal with it.
Adding a small delay solves the problem and doesn't affect results on
other platforms.

Signed-off-by: Fabrice DJIATSA <fabrice.djiatsa-ext@st.com>