Includes a memory leak in the checkqueue unit test (but not in
the actual code). WE really need to switch our checkqueue to use
std::unique_pointer rather than bare pointers. But this would be
invasive enough that I want to do it in a followup PR.
Also pretty-much disable the validation_flush_cache unit test.
This is a stupid and irritating test which tries to unit-test
exact memory usage of std containers. It already has at least
one "remove wrong assumptions" update upstream and after many
tries I was unable to change all the magic numbers in a way
that'd consistently pass CI for Elements.
Also adds a couple ubsan suppressions about perfectly-legitimate
conversions of integer types.
