roundtrip test fails due to missing native/libcst/tests/fixtures directory in crate

Hello!

```
running 1 test
test roundtrip_fixtures ... FAILED

failures:

---- roundtrip_fixtures stdout ----
thread 'roundtrip_fixtures' panicked at tests/parser_roundtrip.rs:23:21:
read_dir: Os { code: 2, kind: NotFound, message: "No such file or directory" }
stack backtrace:
0: rust_begin_unwind
1: core::panicking::panic_fmt
2: core::result::unwrap_failed
3: core::ops::function::FnOnce::call_once
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

failures:
roundtrip_fixtures

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

error: test failed, to rerun pass `--test parser_roundtrip`
```

There is no `native` top directory in the libcst 1.5.1 crate fetched from crates.io.