refactor: test directory structure

[charles-cooper]

What I did

How I did it

How to verify it

Commit message

refactor: test directory structure

- consolidate a bunch of different directories
- move files around
- merge `conftest.py` and `base_conftest.py`
- better organization - unit/ vs functional/ tests
- rename old `parser/` and `parser_utils/` directories

there is still more refactoring that could be done, and probably some
files could be merged / specific tests in files could be moved around,
but this commit tried to only touch the directory structure for the sake
of reducing merge conflicts in PRs which are still open

Description for the changelog

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[codecov-commenter]

Codecov Report

Merging #3664 (45c507b) into master (806dd90) will decrease coverage by 35.73%.
Report is 1 commits behind head on master.
The diff coverage is n/a.

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@@             Coverage Diff             @@
##           master    #3664       +/-   ##
===========================================
- Coverage   89.61%   53.89%   -35.73%     
===========================================
  Files          80       80               
  Lines       11343    11343               
  Branches     2553     2553               
===========================================
- Hits        10165     6113     -4052     
- Misses        791     4699     +3908     
- Partials      387      531      +144     

see 64 files with indirect coverage changes

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