JDK 9 RFR of 8176895: (fc) Split java/nio/channels/FileChannel/Transfer.java into smaller tests

[Brian Burkhalter]

Please review this test-only change.

Issue:	https://bugs.openjdk.java.net/browse/JDK-8176895
Patch:	http://cr.openjdk.java.net/~bpb/8176895/webrev.00/

Split the test in three, moving two sub-tests which write 4 GB files to one test, one sub-test which writes a 6 GB file to another test, leaving the remaining sub-tests in the original test. This should further reduce the probability of spurious failures due to a slow filesystem.

Thanks,

Brian