Adding another thought:
I think at this point core Blockly doesn't really provide a testing library for external integrations to leverage for their own tests (as emphasis has been placed on ensuring Blockly itself is well tested, though there are some options like the developer tools which might have incidental benefits for testing). This is definitely something that may have value to add in the future, but what would really help is understanding specific use cases that such work would address. In particular: understanding common setup or assertion flows in applications that integrate with Blockly, and why these would benefit as reusable components.
My hunch is that assertions is probably the area with most possible benefit, but care would need to be taken to ensure this is done in a framework-agnostic way since we don't really want to be prescribing any particular testing framework for applications that use Blockly.
There are no plans currently to commit to this type of work, but if you're able to collect any scenarios where having a solution like this would be beneficial, it would be great to know what those are and have them documented for a time when that work does make sense to do.
Ben