[tests] don't undef NDEBUG for test executables

The explicit -UNDEBUG was introduced in the CMake to meson migration (62900cd1)
but as far as I can tell it was not present in the CMake tests definition.

It makes tests less reliable because it enables only some debug messages but not
all.
For example if emper is build with NDEBUG a test using a Semaphore
will output debug messages from the Semaphore but not from the Runtime.