Add OsSystem support to mdtests (#16518)

## Summary

This PR introduces a new mdtest option `system` that can either be
`in-memory` or `os`
where `in-memory` is the default.

The motivation for supporting `os` is so that we can write OS/system
specific tests
with mdtests. Specifically, I want to write mdtests for the module
resolver,
testing that module resolution is case sensitive. 

## Test Plan

I tested that the case-sensitive module resolver test start failing when
setting `system = "os"`

crates/red_knot_python_semantic/src/semantic_index.rs

@@ -409,7 +409,7 @@ impl FusedIterator for ChildrenIter<'_> {}
 mod tests {
     use ruff_db::files::{system_path_to_file, File};
     use ruff_db::parsed::parsed_module;
-    use ruff_db::system::DbWithTestSystem;
+    use ruff_db::system::DbWithWritableSystem as _;
     use ruff_python_ast as ast;
     use ruff_text_size::{Ranged, TextRange};
 
@@ -440,7 +440,7 @@ mod tests {
         file: File,
     }
 
-    fn test_case(content: impl ToString) -> TestCase {
+    fn test_case(content: impl AsRef<str>) -> TestCase {
         let mut db = TestDb::new();
         db.write_file("test.py", content).unwrap();