Add a script to build passive test vectors from transcripts

[bifurcation]

This PR adds the Node script active_to_passive.js that converts a transcript of a live test into a collection of passive test vectors. The script takes two inputs: A config JSON file of the same format provided to test-runner, and a transcript JSON file of the format output by test-runner (thus node active-to-passive.js config.json transcript.json).

For each test case (combination of clients / encryption / ciphersuite), and for each client that joins via Welcome in that test case, the script emits a passive test vector that covers:

1. The Welcome that adds the client to the group
2. Further commits that the client is instructed to handle in the test script
3. ... until the first time the client emits a message (since it is no longer passive)

For example, consider the following case:

1. Alice creates a group
2. Alice adds Bob to the group
3. Alice adds Charlie to the group
4. Alice sends an empty Commit
5. Bob sends an empty Commit
6. Charlie sends an empty Commit

This would lead to two test vectors, from Bob's and Charlie's perspectives. Bob's test vector would cover steps [2, 3, 4] and Charlie's test vector would cover steps [3, 4, 5].

I have verified that the overall flow works, with the following steps:

• Running the config.json file in there repository with MLSpp as the sole client and collecting the output in transcript.json
• Running node active_to_passive.js config.json transcript.json >passive.json
• Having MLSpp validate passive.json as a passive client test vector

[bifurcation]

Note that this will be broken by #130, so maybe we should hold this PR until after that one.