Tests runner: use Python from install dir, and break if not found

Test runner: On Linux, use Python from the precompiled libraries

CentOS on the buildbot still runs Python 3.6, which is also used for the
unit tests. This means that the tests can't use language features that are
available to Blender itself. And testing with a different version of Python
than will be used by the actual code seems like a bad idea to me.

More thorough parsing to allow order-independent comparison

Output Blender's output in the first contents check.

USD: added very simple unit test for USD exporting

Since we can't import USD yet, this just does some minimal checks of an
exported USDA file:

- Check that USD can find its JSON files, and
- check that the exporter hierarchy is ok, including instancing.