Attempt to fix bug surfacing sometimes in test.
Attempt to fix bug surfacing sometimes in test.

Recently, `test_index_manual_policy_mmap` has been failing on Windows.

The idea addressed by this patch is that we forget to sync the parent
directory with the current implementation of atomic writes.
This was done correctly when we were relying the atomicwrites crate.

*crossing fingers*

tantivy-search/tantivy,tantivy-search/tantivy