Fixes #156 - Update os backend to use new io integration test suite

[funkyshu]

No description provided.

[funkyshu]

This one will take a careful rollout. I'll probably make a candidate tag to test in our repos. The changes are very slight but the biggest change is enforcing that changes don't occur to a file until you Close() it. Clearly, that change has broken a lot of tests where other backends use os.File that I still need to fix.

[codecov-commenter]

Codecov Report

Attention: 11 lines in your changes are missing coverage. Please review.

Comparison is base (02139cf) 71.51% compared to head (50522e8) 71.36%.

Files	Patch %	Lines
backend/os/file.go	67.64%	6 Missing and 5 partials ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #157      +/-   ##
==========================================
- Coverage   71.51%   71.36%   -0.15%     
==========================================
  Files          40       40              
  Lines        4672     4700      +28     
==========================================
+ Hits         3341     3354      +13     
- Misses       1056     1066      +10     
- Partials      275      280       +5     

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