You can't even test on Safari without investing in Apple specific hardware. It's not cross platform and Apple doesn't help devs either. Even MS provider VMs so you could actually test their damn closed browser.
Even the open source Epiphany Web browser from GNOME can roll out updates to the core WebKit faster than Safari. It's not a 1:1 comparison, but it's the best way I can test WebKit from Linux even if Safari lags on releasing.