Add `test1279` to verify that `libcurl.def` lists all exported API
functions found in libcurl headers.
Also:
- extend test suite XML `stdout` tag with the `loadfile` attribute.
- fix `tests/extern-scan.pl` and `test1135` to include websocket API.
- use all headers (sorted) in `test1135` instead of a manual list.
- add options `--sort`, `--heading=` to `tests/extern-scan.pl`.
- add `libcurl.def` to the auto-labeler GHA task.
Follow-up to 2ebc74c36aCloses#11570
