tests: minor cleanup in tests

[romanc]

Description

Minor cleanup in pace tests:

• add __init__.py files where missing
• List -> list and Tuple -> tuple
• NDSL has no Namelist anymore
• cleanup unused import and code in test_init_from_geos.py

How Has This Been Tested?

All good as long as existing tests are still passing.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas: N/A
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules: N/A
• New check tests, if applicable, are included: N/A

[romanc]

Rebased onto develop after conflicts popped up. Found a stale reference to ndsl.Namelist, which were removed with NOAA-GFDL/NDSL#297.