RFR: 8294156: Demo positioning of multiple test windows

[Alexey Ivanov]

The manual test frame does not provide any layout managers for test windows, the user has to implement one. This demonstrates how to position a several test UI windows.

We may include some implementations into the manual test framework in the future.

**This PR is not meant to be integrated.** It just demonstrates the feature of adding multiple test windows to `PassFailJFrame` which is reviewed in #15665.

To play around, you can checkout the PR branch using the commands that bots add.

Path to the tests: `test/jdk/java/awt/Window/8294156`.

Alternatively, you can checkout directly from my fork:


git fetch https://github.com/aivanov-jdk/jdk.git demo-manyTestWindows:demo-manyTestWindows
git checkout demo-manyTestWindows

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Depends on: https://git.openjdk.org/jdk/pull/15665

Commit messages:
 - 8294156: Demo positioning of multiple test windows

Changes: https://git.openjdk.org/jdk/pull/15721/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15721&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8294156
  Stats: 381 lines in 5 files changed: 381 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/15721.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15721/head:pull/15721

PR: https://git.openjdk.org/jdk/pull/15721