Make test_save_image more robust

[fmassa]

This makes the test more robust, as if the tests are run many times in parallel, deleting the file might make it fail sometimes

[codecov-io]

Codecov Report

Merging #1037 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1037   +/-   ##
=======================================
  Coverage   63.37%   63.37%           
=======================================
  Files          65       65           
  Lines        5152     5152           
  Branches      772      772           
=======================================
  Hits         3265     3265           
  Misses       1664     1664           
  Partials      223      223

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