[nrf fromtree] tests: logging: log_backend_uart: Disable backends other than UART

Test expects that there are no other backends enabled and some
may be enabled by default.

Signed-off-by: Krzysztof Chruściński <[email protected]>
(cherry picked from commit c143daf)
Signed-off-by: Dominik Ermel <[email protected]>

Author: nordic-krch

tests/subsys/logging/log_backend_uart/prj.conf

@@ -9,3 +9,8 @@ CONFIG_LOG=y
 CONFIG_LOG_BACKEND_UART=y
 CONFIG_LOG_MODE_IMMEDIATE=y
 CONFIG_LOG_PRINTK=n
+#
+# Disable all potential other default backends
+CONFIG_LOG_BACKEND_NATIVE_POSIX=n
+CONFIG_LOG_BACKEND_RTT=n
+CONFIG_LOG_BACKEND_XTENSA_SIM=n