RFR: 8315317: Add test for JDK-8262518

[Prasanta Sadhukhan]

Added automated test for 8262518:SwingNode.setContent does not close previous content, resulting in memory leak

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Commit messages:
 - jckeck fix
 - 8315317: Add test for JDK-8262518

Changes: https://git.openjdk.org/jfx/pull/1228/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1228&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8315317
  Stats: 137 lines in 1 file changed: 137 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jfx/pull/1228.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1228/head:pull/1228

PR: https://git.openjdk.org/jfx/pull/1228