refactor: test directory structure (vyperlang#3664)

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