RFR: 8307135: java/awt/dnd/NotReallySerializableTest/NotReallySerializableTest.java failed

[Sergey Bylokhov]

The test uses the `DragSource.getDefaultDragSource()` which throws HeadlessException. The test should be marked as headful.

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Commit messages:
 - 8307135: java/awt/dnd/NotReallySerializableTest/NotReallySerializableTest.java failed

Changes: https://git.openjdk.org/jdk/pull/13733/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13733&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8307135
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/13733.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13733/head:pull/13733

PR: https://git.openjdk.org/jdk/pull/13733