chore: ask for test plan when test cases are not applicable (#7344)

[ci skip]

.github/pull_request_template.md

@@ -1,11 +1,15 @@
-### Describe the pull request
+## Describe the pull request
 
 A clear and concise description of what the pull request is about, i.e. what problem should be fixed? 
 
 Link to the issue: <!-- paste the issue link here, or put "n/a" if not applicable -->
 
-### Checklist
+## Checklist
 
 - [ ] I agree to follow the [Code of Conduct](https://go.dev/conduct) by submitting this pull request.
 - [ ] I have read and acknowledge the [Contributing guide](https://github.com/gogs/gogs/blob/main/.github/CONTRIBUTING.md).
 - [ ] I have added test cases to cover the new code.
+
+## Test plan
+
+If no test cases added, please provide your test plan here.