[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

crates/red_knot_python_semantic/src/lib.rs

@@ -10,6 +10,7 @@ pub use module_resolver::{resolve_module, system_module_search_paths, KnownModul
 pub use program::{Program, ProgramSettings, PythonPath, SearchPathSettings};
 pub use python_platform::PythonPlatform;
 pub use semantic_model::{HasType, SemanticModel};
+pub use site_packages::SysPrefixPathOrigin;
 
 pub mod ast_node_ref;
 mod db;