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/data/test1279

@@ -0,0 +1,31 @@
+<testcase>
+<info>
+<keywords>
+source analysis
+CURL_EXTERN
+libcurl.def
+</keywords>
+</info>
+
+#
+# Client-side
+<client>
+<server>
+none
+</server>
+
+ <name>
+Verify libcurl.def against CURL_EXTERN declarations
+ </name>
+
+<command type="perl">
+%SRCDIR/extern-scan.pl --heading=EXPORTS --sort %SRCDIR/..
+</command>
+</client>
+
+<verify>
+<stdout loadfile="%SRCDIR/../libcurl.def">
+</stdout>
+</verify>
+
+</testcase>