Initial tests for alta_open_lambda

[camdroid]

With the upcoming Neon webhook deprecation, we need for the
alta-open-update lambda to be able to handle both the legacy and the new
webhook format. Roughly, this is what the deprecation strategy looks
like:

• Enable new Neon webhooks (already done)
• Update lambda to work on both styles of payload (new or legacy)
• Monitor results, ensure that there are no errors and that the new
  webhooks have the correct behavior.
• Disable legacy webhooks (one at a time) in Neon.
• Monitor results, ensure that operations are happening as expected
  (accounts are actually being created, etc.)
• Update lambda to throw an error for legacy webhook calls, without
  updating Alta.
• (Eventually) Remove code for handling legacy Neon webhooks.

To help with the first step of updating the lambda, I want to write a
test suite to capture the current behavior. The point of this isn't to
catch any new bugs, just to verify what's already happening in the
lambda.

Once we can show what behavior is currently happening under the legacy
webhooks, we can start adding in the new webhook formats and ensuring
that the tests work with those, too, using the legacy flag if needed.

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