test_utils.js contains helper functions, which should be stateless
(good practices). This was not the case with patchWindowGetSelection
and unpatchWindowGetSelection, so instead we keep only the first
function which returns the second one.

From the test POV, instead of this:

    utils.patchWindowGetSelection();
    // ...
    utils.unpatchWindowGetSelection();

It now looks like that:

    var unpatchWindowGetSelection = utils.patchWindowGetSelection();
    // ...
    unpatchWindowGetSelection();