RFR: 8224778: test/jdk/demo/jfc/J2Ddemo/J2DdemoTest.java cannot find J2Ddemo.jar

[Phil Race]

bug: https://bugs.openjdk.java.net/browse/JDK-8224778
webrev : http://cr.openjdk.java.net/~prr/8224778/

With this fix the test can find the demo jar in its traditional place as 
before,
and now also where it is placed by JIB to be invoked from RunTests.gmk

-phil