tests: compile to something sensible in release builds

assert()-based tests make no sense without asserts, so make sure asserts are compiled in, even if the rest of the code has asserts turned off.

Right now, these tests compile to no-ops on autotools release builds and meson b_ndebug=true builds, which makes them a bit of a lie. 🍰

Alternatively, we could disable compilation of these tests when NDEBUG is going to be defined, but that sounds like a lot of work for questionable benefits.