Further improvements to dbt sort order linter

[jeancochrane]

#644 got merged in before we could implement two pieces of feedback:

• Adjusting the exclude attribute of the pre-commit hook to be more precise and only exclude files in a directory matching the string venv/ rather than any file containing the substring venv, which could exist without being in a venv dir (comment)
• Adding a docstring to explain the different cases in which the linter runs, and how it expects arguments (comments)

In the course of implementing these two pieces of feedback, I also noticed that the logic to check for properly sorted tests is only looking at data tests and so is ignoring the unit tests we added in #638. This PR also tweaks that logic so that we are properly linting unit tests.

Sample output including both types of unsorted tests:

$ python dbt/scripts/check_sort_dbt_yaml_files.py dbt/models/default/schema/default.vw_pin_universe.yml
In file: dbt/models/default/schema/default.vw_pin_universe.yml
Top level: models
data_tests in this group are not sorted:
---> default_vw_pin_universe_unique_by_14_digit_pin_and_year
---> default_vw_pin_universe_row_count
----------------------------------------
In file: dbt/models/default/schema/default.vw_pin_universe.yml
unit_tests in this group are not sorted:
---> default_vw_pin_universe_filters_invalid_pins
---> default_vw_pin_universe_class_strips_non_alphanumerics
----------------------------------------

The following files have unsorted tests:
dbt/models/default/schema/default.vw_pin_universe.yml (2)

[dfsnow]

Thanks for the cleanup @jeancochrane 🙏 Sorry for the early merge!