tests/dnf: always specify platform_module_id

As long as this matches the build environment, this does not make
a differenece, but let us not depend on this.

This will be useful when automatically transforming dnf to rpm
pipelines, as the platform_module_id is needed as input to
osbuild-composer's dnf-json tool.

Performed using this script:

```

cat $1 | jq '(.stages[]? | select(.name == "org.osbuild.dnf") | .options.module_platform_id) |= . + "platform:f30"'  | sponge $1
cat $1 | jq '(.build.pipeline.stages[]? | select(.name == "org.osbuild.dnf") | .options.module_platform_id) |= . + "platform:f30"'  | sponge $1
```

Signed-off-by: Tom Gundersen <teg@jklm.no>

samples/base.json

@@ -12,7 +12,8 @@
           "@Core",
           "selinux-policy-targeted",
           "grub2-pc"
-        ]
+        ],
+        "module_platform_id": "platform:f30"
       }
     }
   ]