[jdk11u-dev] RFR: 8307083: Open source some drag and drop tests 3

[Amos Shi]

Backport of [JDK-8307083](https://bugs.openjdk.org/browse/JDK-8307083)

Testing
- Local: Test passed
  - `MissedDragEnterTest.java`: Test results: passed: 1
  - `ModalDialogDeadlockTest.java`: Test results: passed: 1
  - `ModalDialogOnDragDeadlockTest.java`: Test results: passed: 1
  - `ModalDialogOnDropDeadlockTest.java`: Test results: passed: 1
- Pipeline: 
- Testing Machine:

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Commit messages:
 - Backport 1b154e4fd361103936f976db72e04b73aa7b1077

Changes: https://git.openjdk.org/jdk11u-dev/pull/2565/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=2565&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8307083
  Stats: 895 lines in 4 files changed: 895 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk11u-dev/pull/2565.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u-dev.git pull/2565/head:pull/2565

PR: https://git.openjdk.org/jdk11u-dev/pull/2565