Apple: Add unit tests for MaterialX 1.39

[ld-kerley]

• also updates the default version …to build with - this is only intended for testing.

Description of Change(s)

Add some unit tests to exercise the MatX 1.39 support in both USD and Hydra

NOTE : This PR changes the default version for MaterialX in build_usd.py - that is merely to indicate that MaterialX 1.39 is required for these tests to run. That change should likely be made once further production testing has been performed.

Fixes Issue(s)

Checklist

[X] I have created this PR based on the dev branch

[X] I have followed the coding conventions

[X] I have added unit tests that exercise this functionality (Reference: testing guidelines)

[X] I have verified that all unit tests pass with the proposed changes

[X] I have submitted a signed Contributor License Agreement (Reference: Contributor License Agreement instructions)

[ld-kerley]

Note : the atan2 test doesn't run, because there isn't enough type information in Hydra to reconstruct the MaterialX document correctly. We may need to investigate pivoting from relying on the MaterialX node definitions to get access to the port types, to passing them directly through hydra, but I didn't want to do this work without input from Pixar.

[jesschimein]

Filed as internal issue #USD-10364

[jesschimein]

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[jesschimein]

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