[assertandreturntest] Don't redefine already defined NDEBUG
That is, properly #undef it first. See https://gitlab.dune-project.org/core/dune-common/-/jobs/85361:
/builds/core/dune-common/dune/common/test/assertandreturntest.cc:6: warning: "NDEBUG" redefined
#define NDEBUG TEST_NDEBUG
<command-line>: note: this is the location of the previous definition