Update linux.x86 rtest svg reference files

Update linux.x86 rtest svg reference files

Compared old and new images by visual inspection and found no
differences except:

html2_dot.svg: Color names used instead of hex codes which gives
different shades of gray. Seems to conform to SVG color scheme
according to https://www.graphviz.org/doc/info/colors.html. Slightly
different edge contact point.

url_dot.svg: Color names used instead of hex codes which gives
different shades of green and gray. Seems to conform to SVG color
scheme according to https://www.graphviz.org/doc/info/colors.html.

cairo_dot_cairo.svg: Old has white background. New has
transparent. This seems ok since the default bgcolor is <none>
according to https://www.graphviz.org/doc/info/attrs.html#a:bgcolor.