Write integration tests

[dmikusa]

Write up integration tests to validate the fitness of the profile buildpack. These tests should use occam.

Test cases:

• No profile: it should not detect
• Profile with env variables: it should detect & source the profile, setting env variables
• Profile with side-effect: it should detect & source the profile, triggering the side effect

[dmikusa]

Rather than integration tests, I think we should switch this to using this RFC. The main gist of integration testing is to verify detection behavior. This would allow us to verify detection behavior without mocking anything in Profile CNB but not require full builds.