Add or fix test_package for pyside and partio Conan packages

The `partio` Conan package needs a `test_package`
The `pyside` Conan package needs to have its `test_package` re-enabled.

Locate the Conan package definitions for partio and pyside and inspect how their test_package sections are handled elsewhere in the repository. Run the existing package tests first, then add coverage for partio and verify that pyside's test_package runs again; done means both packages pass their package tests.

Written by the indexing model from the issue text.