Call waitForExplicitFinish in test_img_width_height.html.

This fixes an intermittent failure introduced in
<https://github.com/servo/servo/pull/3929>.
auto merge of #3943 : Ms2ger/servo/img_width_height, r=jdm

This fixes an intermittent failure introduced in
<https://github.com/servo/servo/pull/3929>.