RFR: JDK-8307128: Open source some drag and drop tests 4 [v2]

[Alisen Chung]

> added DnD tests

Alisen Chung has updated the pull request incrementally with two additional commits since the last revision:

 - space
 - fix spacing, move check onto EDT

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/13765/files
  - new: https://git.openjdk.org/jdk/pull/13765/files/1c58b757..5adceab6

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13765&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13765&range=00-01

  Stats: 13 lines in 5 files changed: 2 ins; 0 del; 11 mod
  Patch: https://git.openjdk.org/jdk/pull/13765.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13765/head:pull/13765

PR: https://git.openjdk.org/jdk/pull/13765