Keep updated CellRendererInvoker self.path updated - Fixes #4717

Since CellRendererInvoker can set the path at None if a mouse event
is out of the area of the renderer, can break the use of touch,
if happen after the mouse movement. This patch set the path,
in the point_in_cell_renderer test, for the positive case,
solving the issue.

Signed-off-by: Gonzalo Odiard <godiard@sugarlabs.org>