Handbrake is a great tool in general and depending on codec and settings it can utilize a lot of cores. Asus Realbench has it as a component integrated along with gimp image manipulation and some other open source stuff. I like Realbench or Aida 64 for stress testing and Cinebench R15 for a general comparison of computational power.
Benchmarking or stress testing? Handbrake, or rather x264, is good but for stress testing I'd rather use something else, like RealBench which was already mentioned above or the x264 Stress Test v2. x265 is also a pretty good choice as it hammers down on your CPU really hard. You could for example use HWBot's x265 Benchmark. Using the overkill mode multiplier for 4K encoding is going to give you a pretty damn long stress test. However if you want to compare your results to something else, eg benchmark, then use Cinebench R15 or the HWBot x265 benchmark or something similar. They're quite widely used and as such it's easy to compare.