Add integration tests

[nbigaouette]

Add integration test downloading a model from the zoo and validating the inference.

[codecov-commenter]

Codecov Report

Merging #17 into master will increase coverage by 4.21%.
The diff coverage is 59.06%.

@@            Coverage Diff             @@
##           master      #17      +/-   ##
==========================================
+ Coverage   17.33%   21.55%   +4.21%     
==========================================
  Files          15       17       +2     
  Lines         502      529      +27     
==========================================
+ Hits           87      114      +27     
  Misses        415      415              

Impacted Files	Coverage Δ
...untime/src/download/vision/image_classification.rs	0.00% <0.00%> (ø)
onnxruntime/src/error.rs	57.14% <0.00%> (ø)
onnxruntime/src/session.rs	0.00% <0.00%> (ø)
onnxruntime/src/tensor/ort_owned_tensor.rs	0.00% <0.00%> (ø)
onnxruntime/src/tensor/ort_tensor.rs	82.35% <82.35%> (ø)
onnxruntime/src/environment.rs	92.45% <91.30%> (+8.73%)	⬆️
onnxruntime/src/lib.rs	70.58% <100.00%> (+20.58%)	⬆️
onnxruntime/src/memory.rs	78.57% <100.00%> (ø)
onnxruntime/src/tensor/ndarray_tensor.rs	100.00% <100.00%> (ø)
... and 3 more

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