test: silence some unused-value warnings

clang gives some warnings for unused values, like the following:

  clang -MMD -MF ./.rmw2prog.o.d -o rmw2prog.o rmw2prog.c -Wall -g -O3 -I.. -I../include -L.. -lmodel
  nestedpromise.c:14:2: warning: expression result unused [-Wunused-value]
          atomic_load_explicit(&z, memory_order_relaxed); // this is only for schedule control
          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  ../include/impatomic.h:2901:46: note: expanded from:
  #define atomic_load_explicit( __a__, __x__ ) \
                                               ^
  ../include/impatomic.h:85:17: note: expanded from:
                  __r__; })
                  ^~~~~
  1 warning generated.