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_benchmark/benches/red_knot.rs

@@ -106,7 +106,7 @@ fn setup_case() -> Case {
     let system = TestSystem::default();
     let fs = system.memory_file_system().clone();
 
-    fs.write_files(
+    fs.write_files_all(
         TOMLLIB_FILES
             .iter()
             .map(|file| (tomllib_path(file), file.code().to_string())),
@@ -173,7 +173,7 @@ fn benchmark_incremental(criterion: &mut Criterion) {
         assert_diagnostics(&case.db, &result);
 
         case.fs
-            .write_file(
+            .write_file_all(
                 &case.re_path,
                 format!("{}\n# A comment\n", source_text(&case.db, case.re).as_str()),
             )