[tests] lib/libc/gen:assert_test: undefine `NDEBUG`

[ngie]

Summary

In the event NDEBUG was defined, assert would become a no-op,
breaking some of the expectations in assert_test around assert(..)
failing generating a coredump.

Suggested by: kevans
MFC after: 4 days
MFC with: 6f3445006a

[kevans]

Thanks!