hg: jdk/sandbox: Add -Djdk.net.usePlainSocketImpl to important tests

[alan.bateman at oracle.com]

Changeset: f22d38b23756
Author:    alanb
Date:      2019-04-06 16:36 +0100
URL:       http://hg.openjdk.java.net/jdk/sandbox/rev/f22d38b23756

Add -Djdk.net.usePlainSocketImpl to important tests

! test/jdk/java/net/ServerSocket/AcceptCauseFileDescriptorLeak.java
! test/jdk/java/net/ServerSocket/UnreferencedSockets.java
! test/jdk/java/net/Socket/ConnectionReset.java
! test/jdk/java/net/Socket/Timeouts.java
! test/jdk/java/net/Socket/UdpSocket.java
! test/jdk/java/net/Socket/asyncClose/AsyncClose.java
! test/jdk/java/net/ipv6tests/TcpTest.java