[v1.15.x] fabtests/common: Use dummy ft_pin_core on macOS

sched_setaffinity is only available on Linux.

Signed-off-by: Ken Raffenetti <raffenet@mcs.anl.gov>
(cherry picked from commit 3425fa7)

fabtests/common/shared.c

@@ -3623,6 +3623,7 @@ void ft_free_string_array(char **s)
 	free(s);
 }
 
+#ifndef __APPLE__
 static const char *nexttoken(const char *str,  int chr)
 {
 	if (str)
@@ -3676,6 +3677,12 @@ static int ft_pin_core(const char *core_list)
 
 	return sched_setaffinity(0, sizeof(mask), &mask);
 }
+#else
+static int ft_pin_core(const char *core_list)
+{
+    return EXIT_FAILURE;
+}
+#endif
 
 static int ft_parse_pin_core_opt(char *optarg)
 {