tests: drivers: uart_async_api: add tx buffer in nocache memory

Since nucleo_f746zg has NOCACHE_MEM defined
(related to test cases drivers.uart.async_api.nocache_mem
and drivers.uart.async_api.nocache_mem_dt.nucleo_f746zg),
the TX buffer should be placed in a non-cacheable memory region
for the uart_async_var_buf_length testsuite to pass.

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

Author: djiatsaf-st

tests/drivers/uart/uart_async_api/src/test_uart_async.c

@@ -1068,7 +1068,12 @@ static void *var_buf_length_setup(void)
 static void test_uart_async_var_buf(size_t buf_len, size_t tx_len)
 {
 	int ret;
-	uint8_t tx_buffer[VAR_LENGTH_TX_BUF_SIZE];
+
+#if NOCACHE_MEM
+static __aligned(sizeof(void *)) uint8_t tx_buffer[VAR_LENGTH_TX_BUF_SIZE] __used __NOCACHE;
+#else
+static ZTEST_BMEM uint8_t tx_buffer[VAR_LENGTH_TX_BUF_SIZE];
+#endif /* NOCACHE_MEM */
 
 	for (size_t i = 0; i < VAR_LENGTH_TX_BUF_SIZE; ++i) {
 		tx_buffer[i] = tx_len;