Add tests + CI

[mikegerber]

• Tests
• CI

[mikegerber]

Some basic CI functionality is there, but

• only a dummy test for now
• there seems to be no standard on specifying tests in pyproject (setuptools has test_suite but is it deprecated now as setup.py test seems to be deprecated?)
• only runs on release (this is easily fixable)

[mikegerber]

Testing this is a bit tricky:

• Build a local wheel and serve it via HTTP (serve-local-wheel.sh)
• Have a test project and install it using pip --extra-index-url
  • How to make sure it only uses the test-built wheel, not the old one on PyPI?
• Check that the version came from ocrd-tool.json

[mikegerber]

I found a simpler way to test this: Build a test project without build isolation, with setuptools_ocrd installed in the active virtualenv: python3 -m build --no-isolation.

Documented here https://github.com/qurator-spk/setuptools_ocrd/blob/master/README-DEV.md#without-build-isolation

[mikegerber]

I've added a test project in test_project/.