Add output validation to integration tests

[cgardens]

Current Behavior

• Integration tests pass even though they output json that is not valid SingerCatalog, etc.
  • e.g. stripe integration tests were passing even though it was returning an output that violated the SingerCatalog in multiple different ways.

Expected Behavior

• We should validate in our integration tests that the output of our integrations can actually be deserialized into the types we expect.

Steps to Reproduce (though hopefully I'll have this fixed pretty soon at least for this case)

1. Setup a stripe source integration
2. Try to run discover_schema on it

[cgardens]

this was achieved with the standard tests