tests/test-char: Use a proper fallthrough comment

For being able to compile with -Werror=implicit-fallthrough we need
to use comments that the compiler recognizes. Use "fallthrough" instead
of "no break" here.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20201002171343.283426-1-thuth@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>

tests/test-char.c

@@ -70,7 +70,7 @@ static void fe_event(void *opaque, QEMUChrEvent event)
             h->openclose_mismatch = true;
         }
         h->is_open = new_open_state;
-        /* no break */
+        /* fallthrough */
     default:
         quit = true;
         break;