tests/subsys/modem/backends/tty: Ensure it is built with proper C lib

This test itself (not the code under test) needs to be build with the
host C library as it calls into its open/close/read/write APIs.
So let's ensure we build it with it, independently of what the
default C library may be.

Signed-off-by: Alberto Escolar Piedras <[email protected]>

Author: aescolar

tests/subsys/modem/backends/tty/prj.conf

@@ -6,6 +6,8 @@ CONFIG_NO_OPTIMIZATIONS=y
 CONFIG_MODEM_MODULES=y
 CONFIG_MODEM_BACKEND_TTY=y
 
+CONFIG_EXTERNAL_LIBC=y
+
 CONFIG_ZTEST=y
 
 CONFIG_LOG=y