refactor(isthmus): strptime function mapping

[bestbeforetoday]

Avoid code duplication. Also test operator name and operands explicitly instead of relying on Calcite's toString implementation, which might change over time.

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[nielspardon]

Also test operator name and operands explicitly instead of relying on Calcite's toString implementation, which might change over time.

I recall that in the past when I added unit tests that there was a preference to rely on Calcite's explain() / toString() implementation for readability of the tests. I guess we should get some clarity on that. Otherwise I'm fine.

[bestbeforetoday]

I generally avoid using toString since the Javadoc for Object.toString has this API note (my emphasis):

In general, the toString method returns a string that "textually represents" this object. The result should be a concise but informative representation that is easy for a person to read. It is recommended that all subclasses override this method. The string output is not necessarily stable over time or across JVM invocations.

I don't see anything in the Calcite Javadoc that confirms the toString format or that it is stable across releases.

For now I have just reverted to using toString. It can be revisited later if it ever breaks.