RFR: JDK-8225013: sanity/client/SwingSet/src/ScrollPaneDemoTest.java fails on Linux

[Alexandre Iline]

Before the fix the test was failing around 1 time in fifty runs. After the fix the test did not fail in ~ 1k runs.
Additional instrumentation is added to save useful information on the UI state, should the failure happen again.

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Commit messages:
 - A public method to capture the whole screen.
 - 1. Wait for ScrollPaneDemo container to be loaded.

Changes: https://git.openjdk.java.net/jdk/pull/8848/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8848&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8225013
  Stats: 52 lines in 3 files changed: 45 ins; 1 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8848.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8848/head:pull/8848

PR: https://git.openjdk.java.net/jdk/pull/8848