Use `oiiotool` instead of `idiff` for image comparison

[nvmkuruc]

Comparing images can be done via either the OpenImageIO utilities idiff or via oiiotool.

The pip package for OpenImageIO only includes oiiotool. If oiiotool is more broadly available, it seems like it would be helpful to use oiiotool instead of idiff for comparing images via testWrapper. Would there be any drawbacks to making oiiotool the diffing tool for testWrapper? Are there packages or environments where idiff is available but oiiotool is not?

[jesschimein]

Filed as internal issue #USD-11669

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