VOC2007 support test set

[fmassa]

Fixes #1336

Instead of always enabling a test set, we explicitly only allow it for VOC2007.

@vedaldi @pmeier does this sound ok to you?

[codecov-io]

Codecov Report

Merging #1340 into master will decrease coverage by 0.06%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #1340      +/-   ##
==========================================
- Coverage   65.84%   65.77%   -0.07%     
==========================================
  Files          75       75              
  Lines        5785     5791       +6     
  Branches      885      887       +2     
==========================================
  Hits         3809     3809              
- Misses       1710     1716       +6     
  Partials      266      266

Impacted Files	Coverage Δ
torchvision/datasets/voc.py	21.64% <0%> (-1.43%)	⬇️
torchvision/transforms/transforms.py	80.98% <0%> (ø)	⬆️

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