Adding a unit test suite

[GenevieveBuckley]

Adds a unit test suite
Closes #19

This PR:

• Adds a tox.ini configuration file
• Adds a GitHub Actions config to run CI tests
• Tests that pyproject.toml can be parsed without error
• Tests that there are no flake8 errors raised on any of the generated python code
• Tests that all the generated python files can be compiled using py_compile
• Tests that the filename tree structure of the generated app matches what we expect

STILL TO DO:

• Expand to test all GUI frameworks (PySide, PursuedPyBear, etc), not just the default Done!
• Replace os.path functions with pathlib where appropriate

I had assumed adding support for all the GUI frameworks would be as easy as adding a couple more fixtures and @pytest.mark.parametrize to all the tests -- but it seems like you can't pass fixtures into parametrize like that. Any suggestions?

[freakboy3742]

This is a great start. I've flagged a few cleanups (mostly stylistic variations, or conventions that BeeWare projects have that may not be obvious).

As for the parametrization question; You can't use a fixture as a parameter (AFAIK), but you can pass parameters to a fixture:

@pytest.fixture
def tester(tester_arg):
    return tester_arg * 3 + 1

@pytest.mark.parametrize('tester_arg', [0, 1, 2, 3])
def test_mytest(tester):
    assert tester % 2 == 0

So - if you defined a fixture for a "project", and passed in the GUI library (and maybe the license as well) as an argument to the template, you can then parameterize the tests with the parameterized project as a fixture. Since there are multiple tests that need to be parametrized in this way, it might be helpful to pull out the "list of test configurations" as a constant that can be re-used between the individual tests.

[GenevieveBuckley]

I've addressed all the points you made, thank you for the review!

Note that:

• I didn't add licenses other than the default to our test cases, primarily because all that changes is the contents of a single text file & there is no test checking that content currently. So as things stand now, it's a little meaningless.
• The fixture has a function-scope. Maybe that's slightly inefficient but on the whole it's fine because the 15 tests run in under 13 seconds. (That does bug me a little, I wanted to make it class scoped then pack the three tests into a single class & parameterize that. But I couldn't get that working nicely with the argument passing,)

[freakboy3742]

Looks great! Thanks for the contribution!