Future 4/n: test & legacy in test/ folder (#13295)

* move: legacy >> test/

* move: tests >> test/

* rename unittests

* update CI

* tests4pl

* tests_pytorch

* proxi

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* ci

* link

* cli

* standalone

* fixing

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* .

* Apply suggestions from code review

Co-authored-by: Akihiro Nitta <[email protected]>

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* alone

* test -> tests

* Standalone fixes

* ci

* Update

* More fixes

* Fix coverage

* Fix mypy

* mypy

* Empty-Commit

* Fix

* mypy just for pl

* Fix standalone

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Akihiro Nitta <[email protected]>
Co-authored-by: Carlos Mocholí <[email protected]>