RFR: 8298496: IconifyTest fails intermittently on Linux

[Kevin Rushforth]

`IconifyTest` is unstable on Linux, which makes it impossible to rely on it to test the ability to iconify and deiconify a Stage. The reason for the instability is that the stacking order of multiple always-on-top windows on Linux is wrong sometimes. See [JDK-8298499](https://bugs.openjdk.org/browse/JDK-8298499). Using two always-on-top Stages is unnecessary for this test, and the test will be more stable without it. The fix is to create the bottom Stage without setting `alwaysOnTop`, leaving only the top Stage `alwaysOnTop`. Further, the test now waits for the bottom Stage to be shown before creating the top Stage. The call to `topStage.toFront()` is removed as it is unnecessary. It should never have been needed, and its presence suggests that it might have been an attempted workaround for the stacking order problem (now tracked by JDK-8298499 as mentioned previously). Finally, I increased the wait time between operations from 500 msec to 1000 msec, to match what most other similar 
 tests do (this latter helped stability on my old Ubuntu 16.04 system).

With this fix, the test is now stable on Linux (and is still stable on Windows and Mac). It consistently passes on Linux except Ubuntu 22.04, where it now shows a real problem that undecorated/transparent Stages consistently fail to deiconify (the instability of the test was preventing our being able to see that). Given that #915 fixes this newly discovered problem, I won't file a new bug, but rather will add it to that bug.

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Commit messages:
 - adjust sleep time to 1 second
 - 8298496: IconifyTest fails intermittently on Linux

Changes: https://git.openjdk.org/jfx/pull/975/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=975&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8298496
  Stats: 19 lines in 1 file changed: 6 ins; 4 del; 9 mod
  Patch: https://git.openjdk.org/jfx/pull/975.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/975/head:pull/975

PR: https://git.openjdk.org/jfx/pull/975