[BUILD] Migrate AX Unit Tests from CppUnit to GTest

[danrbailey]

A few years ago, we migrated OpenVDB unit tests from CppUnit to GTest:

#875

However, we never got around to migrating the OpenVDB AX unit tests across. The core unit tests were relatively straight-forward to migrate and CppUnit sometimes causes issues because it's very old and is no longer actively maintained (the last release was 2017). We'd like to finally eliminate this dependency by porting the OpenVDB AX unit tests.

https://freedesktop.org/wiki/Software/cppunit

To build these unit tests, make sure that you are building AX and that you enable the AX unit tests (https://github.com/AcademySoftwareFoundation/openvdb/blob/master/CMakeLists.txt#L77).

[tstraubinger]

I will take a look at this today as part of ASWF Dev Days