RFR JDK-8198922 Provide instrumentation for sanity/client/SwingSet/src/ButtonDemoScreenshotTest.java

[Alexandre (Shura) Iline]

Hi,

Could you please take a look on this fix?

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This fix adds some instrumentation to an existing test which is known to fail 
intermittently. 
Bug: https://bugs.openjdk.java.net/browse/JDK-8198922
Webrev: http://cr.openjdk.java.net/~shurailine/8198922/webrev.00/

Thank you.

Shura