Ensure all optional arguments are tested on and off

Some `zstash` optional arguments are either always or never tested. It might be good to add some tests where the always-on options are off and vice-versa.

After #61 merges, these options will be as follows:
Always on:
- `chgrp`: `-R`
- `extract`: `hpss`
- `ls`: `hpss`
- `update`: `hpss`

Always off:
- `check`: `--keep`

Start by locating the existing tests for the zstash commands chgrp, extract, ls, update, and check, then compare their coverage of optional arguments. Add cases that exercise the listed always-on options off and the always-off check --keep option on, with the test suite passing as the definition of done.

Written by the indexing model from the issue text.