Add test suite using pytest-splunk-addon

[sjparkinson]

See https://pytest-splunk-addon.readthedocs.io/en/latest/overview.html.

Add a test suite to ensure the Splunk app configuration processes the Heroku log drain messages correctly.

[sjparkinson]

There's also a bunch of documentation at https://dev.splunk.com/enterprise/docs/developapps/testvalidate. We already make use of Splunk AppInspect.

If we ever add a dashboard, it looks like splunk/splunk-app-testing is a good reference for using the Cypress library for testing it.