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/module_resolver/resolver.rs

@@ -720,7 +720,7 @@ impl<'db> ResolverContext<'db> {
 #[cfg(test)]
 mod tests {
     use ruff_db::files::{system_path_to_file, File, FilePath};
-    use ruff_db::system::DbWithTestSystem;
+    use ruff_db::system::{DbWithTestSystem as _, DbWithWritableSystem as _};
     use ruff_db::testing::{
         assert_const_function_query_was_not_run, assert_function_query_was_not_run,
     };