Change how integration test harness ingest test cases

[thinkingfish]

Currently we need blank lines to separate individual commands inside the same test sequence, and executes each command following the req-rsp-stat order.

It would be more flexible to allow a single stream of mixed req-rsp-stat lines, with empty lines being optional and for visual separation only. For example, we can check stats after sending half of a command and before receiving any responses. This requires changing how tests are loaded, and how the harness executes individual lines.

Because any tests that work with the existing harness will work in the improved version, we can continue to add more test sequences at the meantime.

[thinkingfish]

changes are presented in PR #145