ENH: test download of datasets

[mathurinm]

addresses #102

[codecov-io]

Codecov Report

Merging #104 into master will increase coverage by 5.12%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #104      +/-   ##
=========================================
+ Coverage   71.68%   76.8%   +5.12%     
=========================================
  Files          10      11       +1     
  Lines         671     677       +6     
  Branches       96      96              
=========================================
+ Hits          481     520      +39     
+ Misses        165     126      -39     
- Partials       25      31       +6

Impacted Files	Coverage Δ
celer/datasets/tests/test_datasets.py	100% <100%> (ø)
celer/datasets/libsvm.py	74.24% <0%> (+50%)	⬆️

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