[FLINK-31880][tests] Fixes unit test so it will pass regardless of ti…

[kurtostfeld]

Fixes unit test so it will pass regardless of time zone it is run in. Also use a better testing date.

I submitted this before, I'm resubmitting with issues fixed. This should follow commit and PR style guides.

This is a low priority issue, as the Flink projects only needs unit tests to pass in the CI system, not in other environments in other time zones, but it's helpful to some developers to have this pass, this is very low risk, and this also uses a better test value. The previous code uses new Date(562423) which is 9 minutes after the epoch in UTC, and in UTC-5 time zone it's before the epoch of 1970-01-01, and that's a poor test value.

[flinkbot]

CI report:

• 6e8d876 UNKNOWN

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[kurtostfeld]

@MartijnVisser Can this get merged or should I discard? It's a low priority but low risk small fix.