In practice, I just don't think that's the case. The entire point of a responsive layout is that you don't have to test against every possible resolution - most responsive websites have 2, maybe 3 conceptual layouts. You can get sufficient coverage by separating the layout testing from the site logic - test the layout on desktop with varying window sizes and then test site logic on the major mobile platforms. That usually means 4 desktop browsers with varying window sizes and 3 mobile browsers... not too bad.