RFR: 8314779: [testbug] Add test to all the XYCharts to check if chart components are removed when series is cleared

[Karthik P K]

Added test: `testChartLineAndAreaRemovedOnClearingSeries()` for `StackedAreaChart` to check if the line and fill elements gets removed from the series on clearing the series. This test is added similar to the tests added to `LineChart` and `AreaChart`.
This test is not required for `BarChart`, `BubbleChart` and `ScatterChart` as no node will added to the series in these charts.

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Commit messages:
 - Add test to check if line and area removed on clearing series

Changes: https://git.openjdk.org/jfx/pull/1231/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1231&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8314779
  Stats: 20 lines in 1 file changed: 20 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jfx/pull/1231.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1231/head:pull/1231

PR: https://git.openjdk.org/jfx/pull/1231