Make it easier to work with schema extraction in acceptance tests

[hariso]

Feature description

In the acceptance tests, we have a GenerateRecords method that lets a developer specify which records should be generated by a source. These records are what the acceptance tests will expect.

However, if schema extraction is enabled, the actual records returned by a source in an acceptance test will differ. Currently, there are 2 options:

1. Disable schema extraction
2. Return Avro-encoded records from GenerateRecords (this is a bit cumbersome)

We should make it easier to work with schema extraction in acceptance tests.