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/runtests.pl

@@ -1195,6 +1195,19 @@ sub singletest_check {
     my @stripfile = getpart("verify", "stripfile");
 
     my @validstdout = getpart("verify", "stdout");
+    # get all attributes
+    my %hash = getpartattr("verify", "stdout");
+
+    my $loadfile = $hash{'loadfile'};
+    if ($loadfile) {
+        open(my $tmp, "<", "$loadfile") || die "Cannot open file $loadfile: $!";
+        @validstdout = <$tmp>;
+        close($tmp);
+
+        # Enforce LF newlines on load
+        s/\r\n/\n/g for @validstdout;
+    }
+
     if (@validstdout) {
         # verify redirected stdout
         my @actual = loadarray(stdoutfilename($logdir, $testnum));
@@ -1213,9 +1226,6 @@ sub singletest_check {
             @actual = @newgen;
         }
 
-        # get all attributes
-        my %hash = getpartattr("verify", "stdout");
-
         # get the mode attribute
         my $filemode=$hash{'mode'};
         if($filemode && ($filemode eq "text") && $has_textaware) {