Oh, right. I don't think we want to clear before every draw, no. There may be an argument for clearing at the start of each test, although I think that in that case we'd like to clear with something other than zeroes.
Clearing at the start seems like a good idea. Clearing before every draw will get in the way of things like this, but it's also a bit unfortunate that we currently have some tests that draw the same pixel values multiple times...