Add a unit test suite

[freakboy3742]

#18 fixed a problem with the generated pyproject.toml that was inadvertently introduced when Docker support was added to the template. This fix wouldn't have been needed at all if there was basic pre-merge testing, implemented as a unit test suite.

We should add unit tests template itself. Cookiecutter can be invoked programmatically; so we can run the template with a fixed context and check key properties of the output, like:

• Can pyproject.toml be parsed without error?
• Can app.py be compiled without a syntax error? (we don't have to run it - we can use py_compile to compile the code to detect errors)
• Does flake8 raise any errors on the generated code?

This should be a pytest suite, wrapped with a tox configuration. There should also be a Github Actions configuration that invokes the test suite on every PR. The CI configuration for Briefcase will serve as a starting point for this test setup.

[GenevieveBuckley]

The Driven Data cookicutter for data science has a reasonably extensive test suite, which I found useful to look at. The repository is at: https://github.com/drivendata/cookiecutter-data-science

[freakboy3742]

Fixed by #21.