Support Bash "strict mode" (and fix one unit test)

[djpohly]

📚 Description

Some script authors like to use "unofficial Bash strict mode", enough so that supporting it shows up in comparisons of shell testing frameworks.

Case in point: enabling the -u option uncovered a bug in test_unsuccessful_assert_is_directory_not_writable that is fixed in this PR.

🔖 Changes

• bashunit and its tests now have set -euo pipefail enabled.
• The test mentioned above has been fixed ($a_file was supposed to be $a_directory).
• Arithmetic expansions ((...)), which exit 1 if their result happens to be zero, are fixed with || true.
• The call to check_duplicate_functions does not appear to be intended to cause the script to fail, so it is also silenced with || true.
• Optional parameters are given default-empty behavior using e.g. ${1-}.
• Optional variables are assigned defaults by evaluating "${varname=default}".
• Existence of optional functions is tested explicitly before attempting to execute them. This has the side benefit of ensuring that an external program with the right name (e.g. set_up) will not be run instead.

✅ To-do list

• [x] I updated the CHANGELOG.md to reflect the new feature or fix
• [ ] I updated the documentation to reflect the changes (Not sure there's anything to document, since this fix doesn't require users to do anything different.)

[watermelon-copilot-for-code-review[bot]]

Watermelon AI Summary

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GitHub PRs

• #239 - Support Bash "strict mode" (and fix one unit test) - By djpohly Just now

• #238 - Copy pre-commit hook to real GIT_DIR - By djpohly 22 minutes ago

• #232 - Fix test doubles - By antonio-gg-dev 3 weeks ago

  bashunit is an open repo and Watermelon will serve it for free. 🍉🫶

[Chemaclass]

It seems that it is not working as expected for macos... I will check it out when I find some time these days

[Chemaclass]

I will merge this branch into an internal one with the same name strictmode, so I can check it out and play around to reproduce the bug easier, while keeping the original commits from @djpohly 👍🏼