Fix issue 11000

[dukc]

With the default TZDatabaseDir values, time zones work as designed. However, if one has TZDIR environment variable set without a trailing slash, PosixTimeZone.getInstalledTZNames will turn the time zone names into nonexistent absolute paths, such as /Europe/Rome, failing the test suite.

This should fix it. Also updated an on-site comment a bit while there.

This is #10999 but with a different commit message. I didn't want to edit the commit message in original branch because I have already used the hash of that commit elsewhere.

[dlang-bot]

Thanks for your pull request and interest in making D better, @dukc! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
Please verify that your PR follows this checklist:

• My PR is fully covered with tests (you can see the coverage diff by visiting the details link of the codecov check)
• My PR is as minimal as possible (smaller, focused PRs are easier to review than big ones)
• I have provided a detailed rationale explaining my changes
• New or modified functions have Ddoc comments (with Params: and Returns:)

Please see CONTRIBUTING.md for more information.

---

If you have addressed all reviews or aren't sure how to proceed, don't hesitate to ping us with a simple comment.

Bugzilla references

Your PR doesn't reference any Bugzilla issue.

If your PR contains non-trivial changes, please reference a Bugzilla issue or create a manual changelog.

⚠️⚠️⚠️ Warnings ⚠️⚠️⚠️

• In preparation for migrating from Bugzilla to GitHub Issues, the issue reference syntax has changed. Please add the word "Bugzilla" to issue references. For example, Fix Bugzilla Issue 12345 or Fix Bugzilla 12345.(Reminder: the edit needs to be done in the Git commit message, not the GitHub pull request.)

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + phobos#11001"

[0xEAB]

Is it reasonably possible to add a unittest for this?

[0xEAB]

… else looks good to me.

[dukc]

Is it reasonably possible to add a unittest for this?

It is not going to be a real unit test since it will have to set the environment variable, on top of PosixTimeZone.getInstalledTZNames not being pure in the first place. Rather, it's an integration test as I understand the terminology.

But apart from that. I can do it. After all, it's the version (Posix) test right below the function that failed on my system revealing the issue.