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/ruff_db/src/files.rs

@@ -496,7 +496,7 @@ impl std::error::Error for FileError {}
 mod tests {
     use crate::file_revision::FileRevision;
     use crate::files::{system_path_to_file, vendored_path_to_file, FileError};
-    use crate::system::DbWithTestSystem;
+    use crate::system::DbWithWritableSystem as _;
     use crate::tests::TestDb;
     use crate::vendored::VendoredFileSystemBuilder;
     use zip::CompressionMethod;