Change test: 
By default writing to an existing file must replace the old file and the
file size must be truncated when using OutputStreams.
Changed OutputStream contract:
OutputStream by default must replace previous file truncating contents.
+Updated unit test for this.
Updated starter.
Moved test code to "src". no need to keep test code of test module in 
explicit 'testsrc' directory.
Must use unique directory names. Also forgot to delete old test dirs.
Switch to using "/" as filesystem root as this is now the default.
Added isSymbolicLink() and isHidden().
Added exception.
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