[jdk11u-dev] RFR: 8296934: Write a test to verify whether Undecorated Frame can be iconified or not

[Goetz Lindenmaier]

I backport this for parity with 11.0.20-oracle.

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

Commit messages:
 - Backport 7845b0d7b4193688448b7bf0c427df4976d302e5

Changes: https://git.openjdk.org/jdk11u-dev/pull/1870/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=1870&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8296934
  Stats: 200 lines in 1 file changed: 200 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk11u-dev/pull/1870.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u-dev.git pull/1870/head:pull/1870

PR: https://git.openjdk.org/jdk11u-dev/pull/1870