test: Updated asserts in test_video_reader

[frgfm]

This PR updates all raw assert to the corresponding unittest.TestCase assert method for tests related to torch.ops.video_reader as suggested by #1483.
The specific test was run locally with success, any feedback is welcome!

[codecov-io]

Codecov Report

Merging #1500 into master will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1500      +/-   ##
==========================================
+ Coverage   64.46%   64.49%   +0.03%     
==========================================
  Files          83       83              
  Lines        6421     6428       +7     
  Branches      982      984       +2     
==========================================
+ Hits         4139     4146       +7     
  Misses       1992     1992              
  Partials      290      290

Impacted Files	Coverage Δ
torchvision/transforms/functional_tensor.py	57.14% <0%> (-2.86%)	⬇️
torchvision/models/detection/transform.py	66.92% <0%> (ø)	⬆️
torchvision/models/detection/roi_heads.py	56.14% <0%> (+0.37%)	⬆️
torchvision/ops/boxes.py	100% <0%> (+5.55%)	⬆️

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[fmassa]

Thanks for the PR, but FileNotFoundError is not Python2-compatible, and I think it's not worth the trouble trying to backport it for replacing this assert here.