6963828: TEST_BUG: java/nio/channels/FileTransfer.java takes too long (win)

[Alan Bateman]

One of FileChannel tests takes an age to run on Windows (10 or 11 
minutes in some cases). All the time is spent creating a 4GB file used 
by one of the sub-tests. The change proposed here creates this big file 
as a sparse file. The result is that the test run is now sub-second on 
all machines that I tried. The webrev is here:
    http://cr.openjdk.java.net/~alanb/6963828/webrev/

-Alan.