tests: Silence an unused-but-set-variable warning with GCC 16

Compiling tests with GCC 16 results into this warning:

    gcc -shared -o libmallocintercept.so -fpic -std=gnu99 -Wall -DFAKE_STAT -Werror -Wextra  -U_FILE_OFFSET_BITS -U_TIME_BITS libmallocintercept.c
    libmallocintercept.c: In function ‘free’:
    libmallocintercept.c:79:12: error: variable ‘ptr2’ set but not used [-Werror=unused-but-set-variable=]
       79 |   long int ptr2 = (long int) ptr; ptr2 -= (long int) ptr;
	  |            ^~~~
    cc1: all warnings being treated as errors

The ptr2 variable was added in the past to silence compiler warnings,
probably for the very same reason (commits
75cbe8e507 and
2bfbe19f71). GCC 16 is smarter and
discovers that the ptr2 variables is not needed.

This patch changes the work around to "(void) unused_variable;" idiom
recommended by GCC manual and already used in print_msg().

test/libmallocintercept.c

@@ -76,7 +76,7 @@ void *malloc(size_t size) {
 }
 
 void free(void *ptr) {
-  long int ptr2 = (long int) ptr; ptr2 -= (long int) ptr;
+	(void) ptr; /* unused */
 	print_msg("Called free() on from libmallocintercept...");
 	poke_faketime();
 	print_msg("successfully\n");