The threee tags `<name>`, `</name>` and `<command>` were frequently used
with a leading space that this removes. The reason this habbit is so
widespread in testcases is probably that they have been copy and pasted.
Hence, fixing them all now might curb this practice from now on.
Closes#12028
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