Adding a unit test to verify the use of the `linenothreshold` option
provided by the `highlight` directive [1]. The included document to test
will introduce two sets of `code-block` and `literalinclude` directives
where the first entry generates contents using line numbers and the
second entry generates contents not using line numbers, respectfully.
[1]: https://github.com/sphinx-doc/sphinx/blob/v1.8.4/sphinx/transforms/post_transforms/code.py#L85
Signed-off-by: James Knight <james.d.knight@live.com>
