test: speedup process-title-threadsafe on macOS

This test has been timing out on macOS. Try to avoid that by
reducing the number of iterations run.

PR-URL: libuv#1867
Reviewed-By: Santiago Gimeno <[email protected]>
Reviewed-By: Bert Belder <[email protected]>

Author: cjihrig

test/test-process-title-threadsafe.c

@@ -26,7 +26,7 @@
 #include <string.h>
 
 #ifdef __APPLE__
-# define NUM_ITERATIONS 20
+# define NUM_ITERATIONS 10
 #else
 # define NUM_ITERATIONS 50
 #endif