Move the serializer test suite into samples as a test helper #5576
Description
Is your feature request related to a problem? Please describe.
The serializer test suite was meant to allow external devs to test their round-tripping systems. Eg if they convert exported JSON to protobufs before saving it to their database, and then do the inverse when loading, the test suite was designed to allow them to see if they've covered all cases.
However, this suite still exists in core because there wasn't time to move it at the end of my internship (plus Monica wasn't available to advise)
Describe the solution you'd like
Move the test suite into blockly-samples as a test helper so that external developers can use it to tes their round-tripping systems.