Honor scroll_below_document when jumping to end of document

Honor scroll_below_document when jumping to end of document

While fixing #12491 at 0fed10e4, some band-aids were removed at the
end of `BufferView::scrollToBuffer`. One of them ensured that
scrolling to last paragraph would not leave the last row alone on
screen.

This behavior is actually correct when the scroll_below_document pref is true.

This commit restores the test that avoids the issue.

Of course, this does not fix the real issue, since for example opening
Tutorial.lyx and jumping to "5.3 Errors!" produces the same unwanted
effect.