Tidy some tests and make them runnable

- Comment out print statement from test_xml_parser.py as it seems
  unnecessary.
- Add "if...main" to a couple of tests to make them directly runnable.
Rename README to README.md

- Rename README to README.md so that it displays in Markdown on GitHub.
Reformat README.md in Markdown

[ci skip]
Merge branch 'test-tidying' into dev