DERBY-5824: Disable OSReadOnlyTest when run as privileged user

When running the test as root, it is able to modify the database even
though all database files have been made read-only. Disable the test
in such environments.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1599544 13f79535-47bb-0310-9956-ffa450edef68