What is ~/Library/Developer/XCTestDevices?
Quick answer
~/Library/Developer/XCTestDevices is where Xcode keeps the simulator clones it makes to run tests in parallel. It's empty on many Macs (it was on ours) and can grow if you run parallel tests a lot. It's safe to empty when no tests are running; Xcode makes new clones when it needs them.
Check its size
du -sh ~/Library/Developer/XCTestDevicesCan I delete it?
Yes, with Xcode quit and no tests running:
rm -rf ~/Library/Developer/XCTestDevices/*Your regular simulators live in ~/Library/Developer/CoreSimulator/Devices and aren't affected.
Questions
Why is XCTestDevices empty?
It only fills when you run tests in parallel on simulators.
Are these my simulators?
No. They are temporary copies for testing. Your simulators are in CoreSimulator/Devices.