Fix incorrect date format pattern in unit test

[Marcono1234]

Fixes #1788

Regarding the change h ('clock-hour-of-am-pm (1-12)') to H ('hour-of-day (0-23)'):
Maybe the usage of h was intended before, since it is also using the US locale, however then maybe it should have included the am/pm marker?

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[ben-manes]

The static analyzers prefer an explicit locale. This is just for debugging the test so I think it’s okay. Thanks!