Unit tests for /remind (and clock mocking)

[Zabuzard]

Overview

The /remind command should be covered by unit tests.

This requires clock mocking though, which we should introduce with this as well.

Hints

Fortunately, Java is well equipped for clock mocking. Its just that it requires a change on all time usages in the whole code base. Like, instead of Instant.now(), you would write Instant.now(clock). And then you need to provide a clock at an accessible place that you can mock (similar to Config for example).

[Zabuzard]

Naturally blocked by #373

[Zabuzard]

unblocked, pr has been merged.

[github-actions]

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