Using $write_file in several testcases with the same filename makes skivvy unable to cleanup tempfiles

[hyrfilm]

If several test-cases write to the same file, using $write_file skivvy can't clean up the temporary files.
In general, it's a bad idea to write to the same file because the result would be over-written. But skivvy shouldn't crash like this. Most likely because it keeps the files to clean up in a list and not a set.

This stacktrace below is where several calls have used a $write_file set to fixture_db.txt:

  File "/usr/local/bin/skivvy", line 8, in <module>
    sys.exit(run_skivvy())
  File "/usr/local/lib/python2.7/site-packages/skivvy/skivvy.py", line 226, in run_skivvy
    result = run()
  File "/usr/local/lib/python2.7/site-packages/skivvy/skivvy.py", line 207, in run
    file_util.cleanup_tmp_files()
  File "/usr/local/lib/python2.7/site-packages/skivvy/util/file_util.py", line 32, in cleanup_tmp_files
    os.remove(filename)
OSError: [Errno 2] No such file or directory: 'fixture_db.txt'```