Set up integration/behavioral tests

[tarkatronic]

Feature Request

Is your feature request related to a problem? Please describe.

Some of the unit tests for this project are a bit obtuse and far from ideal. They do not fully encapsulate "real world" scenarios, and will have to be wholly rewritten for large changes, such as #122. It would be best to have a set of behavioral tests to ensure that this project behaves as we expect, from an end user's perspective, when run against an actual real life repository.

Describe the solution you'd like

• Create a github.com/godaddy/tartufo-integration repository
• Pre-seed the repository with some bad and some good commits
• Populate several different config files in the repository
• Build a test suite utilizing behave to run tartufo against this repository in a number of different ways

Describe alternatives you've considered

The alternative is our current state, which is no integration tests, which sucks.

Teachability, Documentation, Adoption, Migration Strategy

This should mainly involve an update to the contributing docs, asking users to write new Behave tests when behavior is changed.