ci(smoke): make custom modules emit text so the bar is not empty

The static custom modules used `exec: echo` (empty output), so the modules
rendered nothing and the bar strip was blank. Have exec print the label text
and use the default `{}` format.

test/smoke/config.jsonc

@@ -9,18 +9,15 @@
     "modules-center": ["custom/center"],
     "modules-right": ["custom/right"],
     "custom/left": {
-        "format": "Waybar",
+        "exec": "echo Waybar",
-        "exec": "echo",
         "interval": "once"
     },
     "custom/center": {
-        "format": "CI smoke test",
+        "exec": "echo 'CI smoke test'",
-        "exec": "echo",
         "interval": "once"
     },
     "custom/right": {
-        "format": "12:34",
+        "exec": "echo 12:34",
-        "exec": "echo",
         "interval": "once"
     }
 }

test/smoke/modules-config.jsonc

@@ -12,8 +12,7 @@
     "memory": {},
     "disk": {},
     "custom/hello": {
-        "format": "ok",
+        "exec": "echo ok",
-        "exec": "echo",
         "interval": "once"
     }
 }