8231254: (fs) Add test for macOS Catalina changes to protect system software

[Brian Burkhalter]

https://bugs.openjdk.java.net/browse/JDK-8231254
http://cr.openjdk.java.net/~bpb/8231254/webrev.00/

This test verifies that “/“ is read-only and any existing directories in /usr/share/firmlinks are not.

Probably testing other APIs on well known files in the read-only volume should be added as well.

Thanks,

Brian