Do not verify thumbnail URLs by default

Partially reverts cca80fe611 and 0ba692acc8

Unless `--check-formats` is specified, this causes yt-dlp to return incorrect thumbnail urls.
See https://github.com/yt-dlp/yt-dlp/issues/340#issuecomment-877909966, #402

But the overhead in general use is not worth it

Closes #694, #725

yt_dlp/YoutubeDL.py

@@ -2095,25 +2095,14 @@ class YoutubeDL(object):
                 t.get('url')))
 
             def thumbnail_tester():
-                if self.params.get('check_formats'):
-                    test_all = True
-                    to_screen = lambda msg: self.to_screen(f'[info] {msg}')
-                else:
-                    test_all = False
-                    to_screen = self.write_debug
-
                 def test_thumbnail(t):
-                    if not test_all and not t.get('_test_url'):
-                        return True
-                    to_screen('Testing thumbnail %s' % t['id'])
+                    self.to_screen(f'[info] Testing thumbnail {t["id"]}')
                     try:
                         self.urlopen(HEADRequest(t['url']))
                     except network_exceptions as err:
-                        to_screen('Unable to connect to thumbnail %s URL "%s" - %s. Skipping...' % (
-                            t['id'], t['url'], error_to_compat_str(err)))
+                        self.to_screen(f'[info] Unable to connect to thumbnail {t["id"]} URL {t["url"]!r} - {err}. Skipping...')
                         return False
                     return True
-
                 return test_thumbnail
 
             for i, t in enumerate(thumbnails):
@@ -2123,7 +2112,7 @@ class YoutubeDL(object):
                     t['resolution'] = '%dx%d' % (t['width'], t['height'])
                 t['url'] = sanitize_url(t['url'])
 
-            if self.params.get('check_formats') is not False:
+            if self.params.get('check_formats'):
                 info_dict['thumbnails'] = LazyList(filter(thumbnail_tester(), thumbnails[::-1])).reverse()
             else:
                 info_dict['thumbnails'] = thumbnails

yt_dlp/extractor/common.py

@@ -233,7 +233,6 @@ class InfoExtractor(object):
                         * "resolution" (optional, string "{width}x{height}",
                                         deprecated)
                         * "filesize" (optional, int)
-                        * "_test_url" (optional, bool) - If true, test the URL
     thumbnail:      Full URL to a video thumbnail image.
     description:    Full video description.
     uploader:       Full name of the video uploader.

yt_dlp/extractor/youtube.py

@@ -2699,21 +2699,18 @@ class YoutubeIE(YoutubeBaseInfoExtractor):
         # The best resolution thumbnails sometimes does not appear in the webpage
         # See: https://github.com/ytdl-org/youtube-dl/issues/29049, https://github.com/yt-dlp/yt-dlp/issues/340
         # List of possible thumbnails - Ref: <https://stackoverflow.com/a/20542029>
-        hq_thumbnail_names = ['maxresdefault', 'hq720', 'sddefault', 'sd1', 'sd2', 'sd3']
-        # TODO: Test them also? - For some videos, even these don't exist
-        guaranteed_thumbnail_names = [
+        thumbnail_names = [
+            'maxresdefault', 'hq720', 'sddefault', 'sd1', 'sd2', 'sd3',
             'hqdefault', 'hq1', 'hq2', 'hq3', '0',
             'mqdefault', 'mq1', 'mq2', 'mq3',
             'default', '1', '2', '3'
         ]
-        thumbnail_names = hq_thumbnail_names + guaranteed_thumbnail_names
         n_thumbnail_names = len(thumbnail_names)
 
         thumbnails.extend({
             'url': 'https://i.ytimg.com/vi{webp}/{video_id}/{name}{live}.{ext}'.format(
                 video_id=video_id, name=name, ext=ext,
                 webp='_webp' if ext == 'webp' else '', live='_live' if is_live else ''),
-            '_test_url': name in hq_thumbnail_names,
         } for name in thumbnail_names for ext in ('webp', 'jpg'))
         for thumb in thumbnails:
             i = next((i for i, t in enumerate(thumbnail_names) if f'/{video_id}/{t}' in thumb['url']), n_thumbnail_names)