It looks like Kotlin might simplify writing tests:
1) Calcite tests often create expressions (linq4j, rex, sql, etc), and the order of elements is "backwards".
For instance, "x AND (y OR z)" becomes and(x, or(y, z)) at best. Writing and updating such code is a bit tedious. It seems like AND and OR could be infix functions (see https://kotlinlang.org/docs/reference/functions.html#infix-notation )
2) extension functions Calcite tests often tend to create DSLs for testing (e.g. CalciteAssert, Tester, and so on). The idea there is to enable fluent APIs and somehow tame the complexity. The problem there is Java is not that suitable for building DSLs.
Extension methods in Kotlin allow to "add a method to existing class", and it might be helpful for cases like parser.parse("...").assertConvertsTo("...") where assertConvertsTo is an extension method (in Java it could be a static method in CalciteAssert class)
3) data classes. Apparently, Calcite deals with data, and data classes could help here as well.
5) multiline string literals
I think it would be much better to co-locate SqlToRel test code and its expected results, so one can see the test code and expectations.
6) Re Checkstyle: there's a standard code style for Kotlin (and it can be verified automatically), however I am not sure we could configure it in the way we have Checkstyle rules. Calcite uses parenthesis a lot, and I am not sure how Kotlin would deal with it.
It looks like adding Kotlin as a <scope>test</scope> should not be a problem, so I wonder if that is feasible.
PS. Using Kotlin for regular Calcite code is a different story, and I am not sure I want to open that discussion (well, I would love to, yet it might be a major change with ripples here and there). I just think it should be safer to try writing some TEST code for Calcite in Kotlin, then evaluate it for other cases (if necessary at all).