I'm unclear how this works. Unless you have a depth texture you can't read the depth buffer so it seems like to test you'd need to render once with gl_FragDepthExt filling in the depth buffer. Then draw again a normal quad (no gl_FragDepthExt) above/at/below what you expected the depth buffer to be and then check. Example If you clearAndDraw a red quad at gl_FragDepthExt = 0.5 then a green normal quad at 0.2 you should see red, then draw green again at 0.7 you should se green. These can be checked with wtu.checkRect.

I'm not sure you need the variation. You could just set gl_FragDepthExt to a uniform to know that it's working