what Paul O'B is saying, the box model height dimension for the relative_test2 div is computing as a zero value in your case. why? by using absolute positioning the elements are taken out of the normal flow. the normal flow being: boxes in boxes in boxes, like matroska dolls. normally, in html, putting boxes in a box (two divs in a div) makes the containing box to expand to contain the other boxes.
putting float or absolute on a child element, makes the parent disregard the box model of it when computing its own.
the clearing mechanism makes the box model for the parent acknowledge the height dimension in the box model for the floating children and adjust.
on the side note, your layout will probably be possible replacing absolute positioning with floating and some additional tinkering.