Use hard-coded values for paddings and margins

This ensures that the resize tests do not depend on the values set in
the CSS files.

Note that this change causes a test to fail with Firefox, but not with
PhantomJS. This is due to a difference in the starting width used by
Firefox and by PhantomJS, and it will be fixed in a following commit.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>