add --dry-short-short/-D and --search-short-S options, clean up implementation, add unit tests

This should finish https://github.com/hpcugent/easybuild-framework/pull/781, built on top of https://github.com/hpcugent/easybuild-framework/pull/786.

Other side-effects:
* `--search` now cooperates with `--robot`
* add unit test for `--dry-run`
* output of `--search` and `--dry-run` are aligned