Tests: Bluetooth: Tester: improve stability by Flow Control and UARTE

Some tests may be unstable because of missing data from BTP frames.
Enabling Flow Control fixes the issue, but seems to work reliably
only along with UARTE.

Signed-off-by: Krzysztof Kopyściński <[email protected]>

Author: KKopyscinski

tests/bluetooth/tester/nrf52840dk_nrf52840.overlay

@@ -7,7 +7,8 @@
 };
 
 &uart0 {
-	compatible = "nordic,nrf-uart";
+	compatible = "nordic,nrf-uarte";
 	current-speed = <115200>;
 	status = "okay";
+	hw-flow-control;
 };