Add client_dir argument and refactor test_import.py

This PR:
- adds an extra `--clientdir` argument to `bin/omero import` allowing to control the client dir (defaulting to `self.ctx.dir`/`lib`/`client`). Apart from allowing to use `cli.invoke()` in the OmeroPy integration tests, @joshmoore pointing out this feature may have other usages when using the CLI with e.g. an upgrade version of Bio-Formats /cc @ximenesuk
- converts `test_import.py` to use `cli.invoke()` (using `--clientdir`). Goals are:

  - to minimize interference with the `library.py` logic
  - to handle the `stderr/stdout` of the `import` command directly in the tests via `capfd` 
  - to simplify the object retrieval logic by parsing the stderr