RFR 8021883 j.u.Random/RandomStream.java test needs more robust timeout duration

[Paul Sandoz]

Hi,

http://cr.openjdk.java.net/~psandoz/tl/JDK-8021883-random-stream-test/webrev/

This patch updates tests for ThreadLocalRandom streams and increases the 
timeout duration. Although the original should be enough for most cases in 
certain virtualized and emulated environments it might not be. I also took the 
liberty of consolidating the code and sprinkling stream and CompletableFuture 
dust on things.

Paul.