RFR: 8294156: Demo positioning of multiple test windows [v2]

[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

Alexey Ivanov has updated the pull request incrementally with one additional commit since the last revision:

  8294156: Additional layouts for vertically positioned test windows

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/15721/files
  - new: https://git.openjdk.org/jdk/pull/15721/files/b8f14f92..4c0e4d09

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=15721&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=15721&range=00-01

  Stats: 133 lines in 5 files changed: 105 ins; 21 del; 7 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