[ie/youtube] Implement external n/sig solver (#14157)

Closes #14404, Closes #14431, Closes #14680, Closes #14707

Authored by: bashonly, coletdjnz, seproDev, Grub4K

Co-authored-by: coletdjnz <coletdjnz@protonmail.com>
Co-authored-by: bashonly <bashonly@protonmail.com>
Co-authored-by: sepro <sepro@sepr0.com>

test/test_jsc/conftest.py

@@ -0,0 +1,60 @@
+import re
+import pathlib
+
+import pytest
+
+import yt_dlp.globals
+from yt_dlp import YoutubeDL
+from yt_dlp.extractor.common import InfoExtractor
+
+
+_TESTDATA_PATH = pathlib.Path(__file__).parent.parent / 'testdata/sigs'
+_player_re = re.compile(r'^.+/player/(?P<id>[a-zA-Z0-9_/.-]+)\.js$')
+_player_id_trans = str.maketrans(dict.fromkeys('/.-', '_'))
+
+
+@pytest.fixture
+def ie() -> InfoExtractor:
+    runtime_names = yt_dlp.globals.supported_js_runtimes.value
+    ydl = YoutubeDL({'js_runtimes': {key: {} for key in runtime_names}})
+    ie = ydl.get_info_extractor('Youtube')
+
+    def _load_player(video_id, player_url, fatal=True):
+        match = _player_re.match(player_url)
+        test_id = match.group('id').translate(_player_id_trans)
+        cached_file = _TESTDATA_PATH / f'player-{test_id}.js'
+
+        if cached_file.exists():
+            return cached_file.read_text()
+
+        if code := ie._download_webpage(player_url, video_id, fatal=fatal):
+            _TESTDATA_PATH.mkdir(exist_ok=True, parents=True)
+            cached_file.write_text(code)
+            return code
+
+        return None
+
+    ie._load_player = _load_player
+    return ie
+
+
+class MockLogger:
+    def trace(self, message: str):
+        print(f'trace: {message}')
+
+    def debug(self, message: str, *, once=False):
+        print(f'debug: {message}')
+
+    def info(self, message: str):
+        print(f'info: {message}')
+
+    def warning(self, message: str, *, once=False):
+        print(f'warning: {message}')
+
+    def error(self, message: str):
+        print(f'error: {message}')
+
+
+@pytest.fixture
+def logger():
+    return MockLogger()