This commit is a followup of D7649: Link C/C++ unit tests into single executable, and ports the USD tests to the new testing approach. It moves test code from tests/gtests/usd into source/blender/io/common and source/blender/io/usd, and adjusts the use of namespaces to be consistent with the other tests.

I decided to put one test into io/usd/tests, instead of io/usd/intern. The reason is that this test does not correspond with a single file in that directory; instead, it tests Blender's integration with the USD library itself.

There are two new CLI arguments for the big test runner:

• --test-assets-dir, which points to the lib/tests directory in the SVN repository. This allows unit tests to find test assets.
• --test-release-dir, which points to bin/{BLENDER_VERSION} in the build directory. At the moment this is only used by the USD test.

The CLI arguments are automatically passed to the test runner when using ctest. When manually running the tests, the arguments are only required when there is a test run that needs them.

For more info about splitting some code into 'common', see rB084c5d6c7e2cf8.

No functional changes to the tests themselves, only to the way they are built & run.