Integration test: Add more complex test cases that leverage seqtest.Generator

[sravotto]

It would be useful to have end-to-end integration tests that leverage seqtest.Generator.
This will generate a sequence of mutations that can be applied into the source. The integration test framework has means to set up the end-to-end environment that will be able to replicate the mutations into the target database. The seqtest,Generator already has code to validate that the data in the target tables has maintained the correct order of updates.

[bobvawter]

The working plan is to expand the seqtest.Generator API to be able to write into the integration-test source database. We can leverage it for the cdc/server and other xylogical frontends.