Port test_datasets_utils to pytest

[AnirudhDagar]

Fixes #4063

@NicolasHug apologies for the delay. Please note the following main changes (might be helpful in reviewing the PR):

• test_detect_file_type is parametrized over file, expected
• test_detect_file_type_incompatible now represents and clubs the following three different tests:
  • test_detect_file_type_no_ext
  • test_detect_file_type_unknown_compression
  • test_detect_file_type_unknown_partial_ext
• test_decompress is parametrized over extension and makes use of _COMPRESSED_FILE_OPENERS dict defined in torchvision.datasets.utils. This also clubs following tests into one:
  • test_decompress_bz2
  • test_decompress_gzip
  • test_decompress_lzma
• test_extract_archive_defer_to_decompress is parametrized over extension and remove_finished and makes use of pytest-mock. Note the original copy required to avoid mocked utils._decompress function in the final assert.
• test_extract_tar is parametrized over extension, mode combining test_extract_tar and test_extract_tar_xz into one.

[AnirudhDagar]

#4113 makes the decompression maps public. I'll update (_COMPRESSED_FILE_OPENERS -> COMPRESSED_FILE_OPENERS ) this PR when that is merged.

[NicolasHug]

Thanks a lot @AnirudhDagar , this looks great!

I just have one comment below which hopefully @pmeier can confirm

[AnirudhDagar]

Some tests in the CI are failing but they seem unrelated to this PR.
Ps. Mentioning #4098 here since it adds some tests for rar extraction support.

[NicolasHug]

Thanks a lot @AnirudhDagar !!