Disable decryption in tests for the native HLS downloader

[kevinoconnor7]

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Description of your pull request and other information

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.

Currently tests that want to do MD5 verification of the download will encounter the error:

  File "/Users/kevinoconnor/Development/youtube-dl/youtube_dl/downloader/hls.py", line 182, in real_download
    decrypt_info['KEY'], AES.MODE_CBC, iv).decrypt(frag_content)
  File "/usr/local/lib/python3.7/site-packages/Crypto/Cipher/blockalgo.py", line 295, in decrypt
    return self._cipher.decrypt(ciphertext)
ValueError: Input strings must be a multiple of 16 in length

See: #27621 and #27620