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 2ebc74c36a

Closes #11570

tests/FILEFORMAT.md

@@ -608,7 +608,7 @@ have a text/binary difference.
 If `nonewline` is set, we will cut off the trailing newline of this given data
 before comparing with the one actually received by the client
 
-### `<stdout [mode="text"] [nonewline="yes"] [crlf="yes"]>`
+### `<stdout [mode="text"] [nonewline="yes"] [crlf="yes"] [loadfile="filename"]>`
 This verifies that this data was passed to stdout.
 
 Use the mode="text" attribute if the output is in text mode on platforms that
@@ -620,6 +620,8 @@ before comparing with the one actually received by the client
 `crlf=yes` forces the newlines to become CRLF even if not written so in the
 test.
 
+`loadfile="filename"` makes loading the data from an external file.
+
 ### `<file name="%LOGDIR/filename" [mode="text"]>`
 The file's contents must be identical to this after the test is complete. Use
 the mode="text" attribute if the output is in text mode on platforms that have