JDK 9 RFR of JDK-8080658: Update FindDecoderBugs.java to use random number generator library

[Amy Lu]

sun/nio/cs/FindDecoderBugs.java

This test uses randomness and has been observed to intermittently fail. 
This patch is to update the test to use the random number testing 
library, to better identify the cause of any future failures.

Please review the patch. I also need a sponsor for this change.

bug: https://bugs.openjdk.java.net/browse/JDK-8080658
webrev: http://cr.openjdk.java.net/~amlu/8080658/webrev.00/

Thanks,
Amy