8249703: test/jdk/java/io/File/GetXSpace.java fails on macos

[Brian Burkhalter]

On macOS, the number of 1024 byte blocks appears to be incorrectly calculated by 'df' using integer division by 2 of the number of 512 byte blocks, resulting in a size smaller than the actual value when the number of blocks is odd.

Possibly instead of having the IS_MAC and IS_WIN constants it should use the equivalent methods in jdk.test.lib.Platform?

Thanks,

Brian

https://bugs.openjdk.java.net/browse/JDK-8249703
http://cr.openjdk.java.net/~bpb/8249703/webrev.00/