gdatetime: Unset LC_ALL for the test as well

Merged Ting-Wei Lan requested to merge wip/lantw/gdatetime-unset-lc-all-for-the-test-as-well into master

This is a follow-up to a0c7f854.

In addition LC_MESSAGES, we should remove LC_ALL from the environment as well. Otherwise, LC_ALL overrides LC_MESSAGES, causing the test to fail on FreeBSD when LC_ALL is set to a non-English locale.

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