We've been using a copy of IWL's Maxwell Pro TCP Test Suite.
We've been exploring its capabilities by running it against various TCP/IP stack implementations. We intentionally picked old or open-source implementations.
Here we have the results of a run against an old release of FreeBSD 10 (but one that is still found in many embedded systems.) This run is old and used an old version of Maxwell Pro's TCP Test Suite. Newer versions have more extensive tests.