Merge #13281: test: Move linters to test/lint, add readme

fa3c910bfe test: Move linters to test/lint, add readme (MarcoFalke)

Pull request description:

  This moves the checks and linters from `devtools` to a subfolder in `test`. (Motivated by my opinion that the dev tools are mostly for generating code and updating the repo whereas the linters are read-only checks.)

  Also, adds a readme to clarify that checks and linters are only meant to prevent bugs and user facing issues, not merely stylistic preference or inconsistencies. (This is motivated by the diversity in developers and work flows as well as existing code styles. It would be too disruptive to change all existing code to a single style or too burdensome to force all developers to adhere to a single style. Also note that our style guide is changing, so locking in at the wrong style "too early" would only waste resources.)

Tree-SHA512: 9b10e89f2aeaf0c8a9ae248aa891d74e0abf0569f8e5dfd266446efa8bfaf19f0ea0980abf0b0b22f0d8416ee90d7435d21a9f9285b66df43f370b7979173406

test/functional/README.md

@@ -20,6 +20,8 @@ don't have test cases for.
 - Where possible, try to adhere to [PEP-8 guidelines](https://www.python.org/dev/peps/pep-0008/)
 - Use a python linter like flake8 before submitting PRs to catch common style
   nits (eg trailing whitespace, unused imports, etc)
+- See [the python lint script](/test/lint/lint-python.sh) that checks for violations that
+  could lead to bugs and issues in the test code.
 - Avoid wildcard imports where possible
 - Use a module-level docstring to describe what the test is testing, and how it
   is testing it.