RFR: 8316627: JViewport Test headless failure

[Damon Nguyen]

Converted test needs headful tag added to avoid failures.

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Commit messages:
 - Add headful tag

Changes: https://git.openjdk.org/jdk/pull/15847/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15847&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8316627
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/15847.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15847/head:pull/15847

PR: https://git.openjdk.org/jdk/pull/15847