tests: fix flaky radius tests by waiting for server readiness

All six radius tests used a fixed sleep 4 after starting radiusd and
ocserv.  On slow or ASAN-instrumented hosts (CentOS 10 CI) this is
insufficient: freeradius with -xx debug logging takes longer than 4
seconds to load its modules, and even after binding UDP 1812 it continues
initializing its user database before it can process auth requests.

Signed-off-by: Nikos Mavrogiannopoulos <n.mavrogiannopoulos@gmail.com>

tests/radius-multi-group-comma

@@ -91,7 +91,8 @@ fi
 
 ${CMDNS2} ${SERV} -p ${PIDFILE} -f -c ${CONFIG} ${DEBUG} & PID=$!
 
-sleep 4
+wait_file_contents "${RADIUSLOG}" "Ready to process requests" 30
+wait_ns_port t ${PORT}
 
 GROUP=group4
 echo " * Tests OU= comma-separated groups ($GROUP)"