Ensure --regions CLI option can be used twice

[khaeru]

As raised by @adrivinca. This was previously-untested behaviour that the "water-ix" CLI command relied on; changes in #137 caused it to no longer work.

The PR restores the original behaviour by refining the code passed to exec_cb(), and adds a test to confirm.

How to review

• Read message_ix_models/util/click.py in the diff.
• Note the CI checks all pass, especially the newly-added .tests.util.test_click.test_regions().

PR checklist

• [x] Continuous integration checks all ✅
• [x] Add or expand tests; coverage checks both ✅
• ~Add, expand, or update documentation.~ N/A
• ~Update doc/whatsnew.~

[adrivinca]

thanks a lot. all looks fine to me. Also the test.