ruff

ac5d220d - [red-knot] Fix `python` setting in mdtests, and rewrite a `site-packages` test as an mdtest (#17222)

[red-knot] Fix `python` setting in mdtests, and rewrite a `site-packages` test as an mdtest (#17222) ## Summary This PR does the following things: - Fixes the `python` configuration setting for mdtest (added in https://github.com/astral-sh/ruff/pull/17221) so that it expects a path pointing to a venv's `sys.prefix` variable rather than the a path pointing to the venv's `site-packages` subdirectory. This brings the `python` setting in mdtest in sync with our CLI `--python` flag. - Tweaks mdtest so that it automatically creates a valid `pyvenv.cfg` file for you if you don't specify one. This makes it much more ergonomic to write an mdtest with a custom `python` setting: red-knot will reject a `python` setting that points to a directory that doesn't have a `pyvenv.cfg` file in it - Tweaks mdtest so that it doesn't check a custom `pyvenv.cfg` as Python source code if you _do_ add a custom `pyvenv.cfg` file for your mock virtual environment in an mdtest. (You get a lot of diagnostics about Python syntax errors in the `pyvenv.cfg` file, otherwise!) - Rewrites the test added in https://github.com/astral-sh/ruff/pull/17178 as an mdtest, and deletes the original test that was added in that PR ## Test Plan I verified that the new mdtest fails if I revert the changes to `resolver.rs` that were added in https://github.com/astral-sh/ruff/pull/17178