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

[Apteryks]

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.

[Apteryks]

(the problematic line/file: https://github.com/Instagram/LibCST/blob/08da127e5434eb42bae5dc71c03363d6a281d4bc/libcst/tests/test_roundtrip.py#L13C1-L13C86)