RFR: JDK-8202790: DnD test DisposeFrameOnDragTest.java does not clean up

[Alisen Chung]

Added a background to the test to prevent the file from dragging onto the desktop and creating an extra file.

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Commit messages:
 - added background to prevent file creation during drag and drop

Changes: https://git.openjdk.java.net/jdk/pull/8370/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8370&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8202790
  Stats: 8 lines in 2 files changed: 7 ins; 1 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8370.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8370/head:pull/8370

PR: https://git.openjdk.java.net/jdk/pull/8370