Add another test to ensure we can call graphics

Author: rocky

mathicsscript/format.py

@@ -59,8 +59,8 @@ def eval_boxes(result, fn, obj, **options):
  elif expr_type in ("System`Graphics", "System`Plot"):
  form_expr = Expression("StandardForm", expr)
  result = form_expr.format(obj, "System`StandardForm")
- svg_str = eval_boxes(result, result.boxes_to_svg, obj)
- if plt and svg2png:
+ if plt and svg2png and hasattr(result, "boxes_to_svg"):
+  svg_str = eval_boxes(result, result.boxes_to_svg, obj)
  temp_png = NamedTemporaryFile(
  mode="w+b", suffix=".png", prefix="mathicsscript-"
  )