Fix integration scripts test

- One of the tests in hdfs_import_export_tests used `xargs` to get
the input from `ls` and subsequently `wc` to count the number of files
in a directory. In case when this kind of check is used to assert when
there are no files, the assertion fails in *nix environment.

This is because in *nix the `xargs` runs even if there are no input and
generates a blank line and `wc` subsequently counts this incorrectly.
To avoid this from happening, `xargs` can be run with `--no-run-if-empty`
option but this is only available in GNU extensions and fails in Mac.

One possible fix is to remove xargs and use `ls -1` to output the results
in single column and use `wc` to count.