Fix missing casts to match printf format specifier in test_atomic

* tests/test_atomic.c (test_and_set_thr): Add missing casts to long
for locked_counter (which is unsigned long) to match printf format
specifier.
Eliminate 'variable set but not used' Cppcheck warnings in test_stack

* tests/test_stack.c (run_one_test): Define "j" local variable only
if VERBOSE.
* tests/test_stack.c (main): Do not define "sum" local variable (and
don not compute the sum) if NO_TIMES.