[ie/LinkedIn] add test for subtitles

Note that the extractr is somewhat broken, so the valus of this test will
have to be adapted when the extractor is fixed.

yt_dlp/extractor/linkedin.py

@@ -13,6 +13,7 @@ from ..utils import (
     strip_or_none,
     mimetype2ext,
     try_get,
+    url_or_none,
     urlencode_postdata,
     urljoin,
 )
@@ -93,6 +94,15 @@ class LinkedInIE(LinkedInBaseIE):
             'description': 'md5:be125430bab1c574f16aeb186a4d5b19',
             'creator': 'Mishal K.'
         },
+    }, {
+        'url': 'https://www.linkedin.com/posts/the-mathworks_2_what-is-mathworks-cloud-center-activity-7151241570371948544-4Gu7',
+        'info_dict': {
+            'id': '2',
+            'ext': 'mp4',
+            'title': 'MathWorks on LinkedIn: What Is MathWorks Cloud Center?',
+            'thumbnail': 'https://media.licdn.com/dms/image/D5610AQFKo9M0zqY2_g/ads-video-thumbnail_720_1280/0/1704988806715?e=2147483647&v=beta&t=0vg-ksOY2KrL_QFlNacCv5Tmuk0tum9FJ_4dlJ56Gyw',
+            'subtitles': 'mincount:1'
+        },
     }]
 
     def _real_extract(self, url):
@@ -104,17 +114,17 @@ class LinkedInIE(LinkedInBaseIE):
         like_count = int_or_none(get_element_by_class('social-counts-reactions__social-counts-numRections', webpage))
         creator = strip_or_none(clean_html(get_element_by_class('comment__actor-name', webpage)))
 
-        video_json = extract_attributes(self._search_regex(r'(<video[^>]+>)', webpage, 'video'))
-        sources = self._parse_json(video_json['data-sources'], video_id)
+        video_attrs = extract_attributes(self._search_regex(r'(<video[^>]+>)', webpage, 'video'))
+        sources = self._parse_json(video_attrs['data-sources'], video_id)
         formats = [{
             'url': source['src'],
             'ext': mimetype2ext(source.get('type')),
             'tbr': float_or_none(source.get('data-bitrate'), scale=1000),
         } for source in sources]
-        subtitles = {}
-        sub_url = video_json.get('data-captions-url')
-        if sub_url:
-            subtitles.setdefault('en', []).append({'url': sub_url, 'ext': 'vtt'})
+        subtitles = {'en': [{
+            'url': video_attrs['data-captions-url'],
+            'ext': 'vtt',
+        }]} if url_or_none(video_attrs.get('data-captions-url')) else {}
 
         return {
             'id': video_id,