Use test images from repo rather than internet

[zhiqwang]

No description provided.

[codecov]

Codecov Report

Merging #149 (62a2fa5) into master (f34194c) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #149   +/-   ##
=======================================
  Coverage   84.49%   84.50%           
=======================================
  Files          12       12           
  Lines         819      813    -6     
=======================================
- Hits          692      687    -5     
+ Misses        127      126    -1     

Flag	Coverage Δ
unittests	84.50% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
test/test_engine.py	94.32% <100.00%> (ø)
test/test_onnx.py	88.60% <100.00%> (+0.65%)	⬆️
test/test_torchscript.py	100.00% <100.00%> (ø)

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