Revert "[tests] don't undef NDEBUG for test executables"

This reverts commit b33ac93c.

Now that logging is done independently from NDEBUG, we can re-enable "undef NDEBUG for test executables". The rationale that we want to undefine NDEBUG for test executable is that we want, even in release builds, asserts in tests enabled. Note that this also applies to asserts in the emper core library, if those are in headers.