Math: improve Matrix4::lookAt() test.
Math: improve Matrix4::lookAt() test.

It now verifies all assumptions about how the resulting matrix should
look like.

MiUishadow/magnum,MiUishadow/magnum,MiUishadow/magnum,MiUishadow/magnum,MiUishadow/magnum,MiUishadow/magnum