Fix 'scope of var can be reduced' cppcheck warnings in check_heap_stats

Fix 'scope of var can be reduced' cppcheck warnings in check_heap_stats
(fix of commit 4e909d2)

Also, print the message about the leak-find mode just once per gctest
execution.

* tests/test.c (INIT_FIND_LEAK): Define macro.
* tests/test.c (GC_COND_INIT): Invoke INIT_FIND_LEAK.
* tests/test.c (run_one_test): Remove print about leak-find mode.
* tests/test.c [!GC_NO_FINALIZATION] (check_heap_stats): Move
still_live and still_long_live local variables to the block of their
usage.