test(sankey): add test case for sankey chart
test(sankey): add test case for sankey chart

add test case for sankey chart of the case when series data key name is nodes。

in the no fixed version, when series data key name is "nodes"（which is supported according to official docs）, the draggable effect is only applied to nodes, and the node is splited from edges
apache/incubator-echarts,apache/incubator-echarts