Add tests

[rikhuijzer]

This PR is the first step to solve #218.

The following statements are now covered by tests:

• build generates, at least, a Markdown file (test all kinds of methods like weave_file and weave_md).
• build on a valid ".jmd" file creates a Markdown file with the generated output and without errors.
• build on a ".jmd" file with a failing assertion gives an error (this is important to check, because it allows adding assertions to the tutorials).