gdb: better test command

Author: cirosantilli

README.adoc

@@ -11798,7 +11798,7 @@ If something goes wrong, re-run the test commands manually and use `--verbose` t
 
 ....
 ./run --arch arm --background --baremetal add --wait-gdb &
-./run-gdb --arch arm --baremetal add --test --verbose
+./run-gdb --arch arm --baremetal add --verbose -- main
 ....
 
 and possibly repeat the GDB steps manually with the usual: