tests: ensure `libcurl.def` contains all exports

tests: ensure `libcurl.def` contains all exports

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 2ebc74c36a19a1700af394c16855ce144d9878e3

Closes #11570