tests/extmod/framebuf1: Add test for no-op fill_rect.

tests/extmod/framebuf1.py

@@ -50,6 +50,7 @@ print('rect', buf)
 
 #fill rect
 fbuf.fill(0)
+fbuf.fill_rect(0, 0, 0, 3, 1) # zero width, no-operation
 fbuf.fill_rect(1, 1, 3, 3, 1)
 print('fill_rect', buf)