RFR 8031961 ProcessBuilder.Basic should use NIO Files to copy files

[Roger Riggs]

Please review this minor test improvement to replace spawning /bin/cp
to copy files with NIO Files.copy.

Webrev:
  http://cr.openjdk.java.net/~rriggs/webrev-process-cp-8031961/

Thanks, Roger

[1]    https://bugs.openjdk.java.net/browse/JDK-8031961