I test manually. I obviously can’t test every possible resolution, so I focus my testing on the extremes: really small window, really wide window, really tall and skinny window, and 2160p full-screen.
Automated testing for UX issues is problematic. I could write automated tests for things like “make sure the enemies spawn off-screen”, “make sure the event my character is reacting to is on-screen”, “make sure all text fits the window without going smaller than a specified font size”, but doing so would be difficult. I can’t write automated tests for more subjective elements like “make sure the player can see far enough that she can reasonably respond to threats in time” and “make sure the text is readable at all resolutions”. So I generally don’t bother with automated testing for UX issues at all.