8202284: FileChannel and FileOutpuStream variants of AtomicAppend should pass silently on macOS >= 10.13

[Brian Burkhalter]

Issue:  https://bugs.openjdk.java.net/browse/JDK-8202284
Patch:  http://cr.openjdk.java.net/~bpb/8202284/webrev.00/

Suite of [1].

Remove AtomicAppend tests from the problem list instead making then pass 
silently for macOS version >= 10.13.

Thanks,

Brian

[1] http://mail.openjdk.java.net/pipermail/core-libs-dev/2018-April/052751.html