Increasing Kernel limits on OS X
On some machines, iOS Simulator tests can run faster once kernel limits are increased. The run-webkit-tests script prints out a warning when it is the case.
In order to increase the limits for number of processes and number of files, save the attached plist files into /Library/LaunchDaemons/ and reboot the system.
On OS X El Capitan, you will also need to disable System Integrity Protection. This is not needed on macOS Sierra.
To check the limits use "ulimit -a" or "launchctl limit maxproc".