The best configurable tool I've found to simulate light to heavy disk access for testing purposes is filebench.
I didn't get filebench-1.5-alpha3 to compile properly. So download an older installer package (filebench 1.4.4) from FileBench ported to Mac OS X or direct link.
Install the app. You may have to disable SIP and - in Sierra - Gatekeeper to do so. The package will be installed to /opt/filebench.
To start the app in interactive mode enter in Terminal.app:
Now choose a workload at the prompt - preconfigured workloads can be found in /opt/filebench/config (e.g. fileserver.conf):
filebench> load fileserver
Set a working directory and other variables (most have default values):
and a run time in seconds:
To create your own workloads visit filebench Wiki and check the WML link.
To "simulate" CPU-load install prime95 and configure it.
In my virtual environment (Sierra - 2 CPUs/4 GB memory) running go_filebench alone already creates a total CPU-load of 70% (go_filebench: ~140%). So I don't have to start prime95 additionally. In your environment you may have to start it though.