fix build issues caused by the order of OIIO includes

[antond-weta]

Had some issues on my Mac with unresolved templates in OIIO. Didn't look much into that, moving the <OpenImageIO/unittest.h> after other headers including the OIIO bits seems to fix that.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.33%. Comparing base (1d95f00) to head (786ef4d).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #198   +/-   ##
=======================================
  Coverage   71.33%   71.33%           
=======================================
  Files          10       10           
  Lines        2348     2348           
  Branches      311      311           
=======================================
  Hits         1675     1675           
  Misses        673      673           

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