RFR: 8236505: Mark jdk/editpad/EditPadTest.java as @headful

[Aleksey Shipilev]

This is a GUI test, and it should be `@headful`.

Additional testing:
 - [x] Test still runs in default, and does not run with `!headful`

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Commit messages:
 - Fix

Changes: https://git.openjdk.java.net/jdk/pull/5544/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5544&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8236505
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5544.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5544/head:pull/5544

PR: https://git.openjdk.java.net/jdk/pull/5544