date.test: defuse a time bomb.

One of these tests fails in the gap between the locale it's run in
changing the year and Europe/Berlin's changeover (caught by our
automated testing, which tends to celebrate every new year by finding
mistakes in tests like this).

Another test has been failing consistently since 2020 because the
default date output format includes the day of the week. Rather than
implement Zeller's Congruence in the shell, explicitly use a format that
doesn't include the day of the week for that test (and reuse the
now-fixed calculation of the current year used by the other
year-sensitive test).

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