Make `tests_util` target compatible with C++20

[Bronek]

Currently tests_util is used as a dependency by tests_pfn and accidentally this works, even though it pulls headers which are designed to work with C++23 only. I am lucky this worked so far, but should not rely on luck for ever.

[Bronek]

Should be addressed in #212 , at least partially - by removing util/static_check.hpp in C++20 mode.

[Bronek]

Should be addressed in #212 , at least partially.

not actually happy with the way this works, because that created two separate tests_util and tests_util_fn where the latter has a dependency on include_fn which is also the thing being tested. That is, we trust the thing being tested to be a part of the test scaffold, and then we proceed to test that very same thing. That's not ideal.

A clean fix is to keep static_check.hpp free of dependency on include_fn; it's mostly just lvalue/rvalue ref-flavor wrappers and a few static_assert helpers. It would cause fair amount of churn in unit tests though, so will need to be addressed in a separate PR (after #212 is delivered)