Add file tests, clean up and simplify

- Test html, idml, dtd, odg/p/s/t File type upload
- Use ZanataRestCaller rather than manually creating projects, versions
- Remove unused imports
- Add a null pointer check on opening files
Is this intended to be lacking the usual html file contents such as `<body>` and the closing tag for html? The contents of `HTM_FILE` and `HTML_FILE` are the same so could go in a variable.