Separate "regular" tests from "extended" tests

[AnyhowStep]

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So, a few functions have have over 200k tests that more-or-less brute force a large amount of inputs.

We should mark these are being "extended" tests that should be run last inside of CI. And these extended tests should not be run during development, unless explicitly requested.

[AnyhowStep]

In particular, the timestampAddXxx() tests. These are prime candidates for being moved to the extended test suite. Maybe have a few dozen cases kept in the regular test suite