Use pytest for integration tests

[ehmatthes]

One of the reasons I haven't done this previously is wanting to keep integration testing separate from unit tests. But pytest really does offer much needed structure for integration testing.

I don't want people to end up running pytest at the root level, and accidentally trigger both unit tests and integration tests. This should be prevented by having a root-level conftest.py that exits when pytest is run at the root level. It can generate a message telling people to cd into either unit_tests/ or integration_tests, and then issue their testing command.

See also #162.

• Add a root-level conftest.py file that prevents root-level use of the pytest command.
• Restructure integration tests using pytest.
• Clarify the distinction between testing the deployment process, and running functionality tests against the deployed project.
• Fix the issue with testing DEBUG against the local version of the project.

[ehmatthes]

Developers will need to have the CLI installed for the platforms they want to run integration test against. It was closed because we don't need any CLIs installed for unit testing, but see the work @rodolfomp123 did on #228. That documentation should be able to be copied into the final documentation for integration tests.