I just open an incognito window in Chrome to test guest permissions. No need to log out at all then. Private browsing in Firefox can now do the same with the latest version, keeping one window logged in and the other as the (private browsing) guest view.
I mean, I too use Incognito / different browser, but for an admin who quickly wants to test it without opening a new window, it seems like an oversight. Without "User" in the title, its slightly misleading if someone doesn't read the description.
I think the new browser window may be the best way around this. People don't seem to understand that test permissions is X's permission set applied to YOU so you get people confused as to what they're expecting to see.