[downloader/hls] Disable decryption in tests (#27660)

Tests truncate the download to 10241 bytes, which is not divisible by 16
and cannot be decrypted. Tests don't really care about the decrypted
content, just that the data they retrieved is the expected data.
Therefore, it's fine to just return the encrypted data to tests.

See: #27621 and #27620

youtube_dl/downloader/hls.py

@@ -172,8 +172,12 @@ class HlsFD(FragmentFD):
                         iv = decrypt_info.get('IV') or compat_struct_pack('>8xq', media_sequence)
                         decrypt_info['KEY'] = decrypt_info.get('KEY') or self.ydl.urlopen(
                             self._prepare_url(info_dict, info_dict.get('_decryption_key_url') or decrypt_info['URI'])).read()
-                        frag_content = AES.new(
-                            decrypt_info['KEY'], AES.MODE_CBC, iv).decrypt(frag_content)
+                        # Don't decrypt the content in tests since the data is explicitly truncated and it's not to a valid block
+                        # size (see https://github.com/ytdl-org/youtube-dl/pull/27660). Tests only care that the correct data downloaded,
+                        # not what it decrypts to.
+                        if not test:
+                            frag_content = AES.new(
+                                decrypt_info['KEY'], AES.MODE_CBC, iv).decrypt(frag_content)
                     self._append_fragment(ctx, frag_content)
                     # We only download the first fragment during the test
                     if test: