Improve the test error reporting

[mordante]

          Thanks! It seems I overlooked the CI failure. I think the error can be improved, but I disagree that this is not useful. If I'd gotten a "typical" assert message I would not have been able to figure out what the error was; it wouldn't have shown 1950-01-01 00:00:00. This value helped me to discover that the `test_indian_kerguelen` test failed. I fully agree that this error does not help those not intimately familiar with this test.

I'll think about a way to improve the error to be more useful.

Originally posted by @mordante in #85619 (comment)