fix test for pytest 4

[jesterhazy]

Description of changes:

fix test failing due to fixture error

Merge Checklist

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• I have added tests that prove my fix is effective or that my feature works (if appropriate)
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[codecov-io]

Codecov Report

Merging #482 into master will decrease coverage by 0.14%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #482      +/-   ##
==========================================
- Coverage   94.02%   93.88%   -0.15%     
==========================================
  Files          57       57              
  Lines        4269     4269              
==========================================
- Hits         4014     4008       -6     
- Misses        255      261       +6

Impacted Files	Coverage Δ
src/sagemaker/local/image.py	88.08% <0%> (-1.89%)	⬇️

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