Send statuses to the API

[ehmicky]

Part of #1223
Fixes #1238.

This sends success and error statuses to the Bitballoon API endpoint.

This PR also adds many tests:

• API reporting under several conditions
• fix the previous tests about "implicit" statuses, as previously discussed with @erezrokah
• API reporting on different types of plugin errors
• "implicit" status not used when a plugin did not complete due to another plugin making the build fail
• the previous tests that were relying on checking the build logs are now also checking the API request
• in CI builds, statuses should be sent to the API but not printed in logs

API testing works by mocking the HTTP request and snapshotting its request body, headers and method. The body is normalized so the snapshot remains stable over time and across machines.