hg: jdk/sandbox: Add test to exercise using custom SocketImpls

[alan.bateman at oracle.com]

Changeset: 88a41734ddbe
Author:    alanb
Date:      2019-02-20 20:55 +0000
URL:       http://hg.openjdk.java.net/jdk/sandbox/rev/88a41734ddbe

Add test to exercise using custom SocketImpls

! src/java.base/share/classes/java/net/AbstractPlainSocketImpl.java
! src/java.base/share/classes/java/net/ServerSocket.java
+ test/jdk/java/net/SocketImpl/CustomSocketImpls.java