tests: make g0 test more robust by waiting for linuxcnc to come up

This should fix the failing test, as discovered here:
#2700 (comment)

tests/motion/g0/test.sh

@@ -23,6 +23,21 @@ wait_for_pin() {
 linuxcnc motion-test.ini &
 linuxcncpid=$!
 
+# let linuxcnc come up
+TOGO=80
+while [  $TOGO -gt 0 ]; do
+    echo trying to connect to linuxcncrsh TOGO=$TOGO
+    if nc -z localhost 5007; then
+        break
+    fi
+    sleep 0.25
+    TOGO=$(($TOGO - 1))
+done
+if [  $TOGO -eq 0 ]; then
+    echo connection to linuxcncrsh timed out
+    exit 1
+fi
+
 wait_for_pin motion.in-position TRUE
 
 echo starting to capture data