[11u] RFR 8243488: Add tests for set/get SendBufferSize and getReceiveBufferSize in DatagramSocket

[Aleksey Shipilev]

Original improvement:
   https://bugs.openjdk.java.net/browse/JDK-8243488
   https://hg.openjdk.java.net/jdk/jdk/rev/cf9358cd555b

Backporting better tests. The conflict for 11u is in removal of the old test (that is now subsumed 
by the new one). Removed that test by hand.

11u variant:
   https://cr.openjdk.java.net/~shade/8243488/webrev.11u.01/

Testing: affected tests on Linux x86_64

-- 
Thanks,
-Aleksey