Function wasmer_wast::wasi_wast::map_host_fs_to_mem_fs

fn map_host_fs_to_mem_fs<'a>(
    fs: &'a dyn FileSystem,
    directory_reader: ReadDir,
    path_prefix: &'a Path,
) -> Pin<Box<dyn Future<Output = Result<()>> + 'a>>

When using virtual_fs::mem_fs, we cannot rely on BASE_TEST_DIR because the host filesystem cannot be used. Instead, we are copying BASE_TEST_DIR to the mem_fs.