Simplify compilation of :core tests with compiler plugin enabled for better E2E testing

In tests we have all sorts of dataframe code. Compiling it with compiler plugin could help us catch bugs earlier. For example, regression causing compiler crash found by user was reproducable in one of the tests.
https://github.com/Kotlin/dataframe/issues/1723

Problem:
Simply enabling plugin, we still have a bunch of errors before we could compile:

Goal: I have no intention to enable plugin by default (yet?), but it would be nice to only have to apply a small patch that enables it and simply run "test" task. So need to go over all compilation errors and fix what's possible

Most of them can be fixed by slightly changing the code of a test itself, in some cases i'll adjust compiler plugin to be not so strict/invasive.

Start with the :core tests and the compilation errors produced when the compiler plugin is enabled; review the linked regression issue #1723 for context. Work through the test and compiler-plugin changes needed without enabling the plugin by default. Done means the test task runs successfully with the plugin enabled.

Written by the indexing model from the issue text.