DeferredFileOutputStream better NPE message and new file permission tests

[garydgregory]

• Add tests to validate owner permission on POSIX.
• Refactor test to check a specific folder for POSIX instead of the default FS.
• DeferredFileOutputStream.thresholdReached(): Throws an NPE with a useful message.
• DeferredFileOutputStream.thresholdReached(): Better local variable name.

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