Omit new unit test

In this case, I think it's fine to omit the new unit test for these specific swizzle patterns, following the approach we use for validating earlier version upgrades.

If there are production examples of materials leveraging these swizzle patterns, it would be fine to include them in our test suite with other version upgrade examples:

https://github.com/AcademySoftwareFoundation/MaterialX/tree/main/resources/Materials/TestSuite/stdlib/upgrade

Signed-off-by: Jonathan Stone <jstone@lucasfilm.com>