Add basic end-to-end and unit tests

[ambergristle]

Describe key service behavior, and flag breaking changes.

What

• End-to-end tests verifying service responds appropriately to both valid and invalid requests:
  • Malformed or missing parameters
  • [Future] Unauthorized requests
• Unit tests to verify key logic or behavior:
  • Signature formatting
  • Parameter validation?
  • Callback construction?

Considerations

• How much can we lean on end-to-end tests?
  • Signature logic seems like the only piece that really benefits from unit testing.

Tests

End-to-end

• happy path: server responds 202 with valid data + faithfully relays to callback
• bad data: server responds 400 if request data is missing or malformed
  • for every parameter
• query params: server ignores query params
• routing: 404s on bad endpoints
• server: timeouts
• unauthorized: [future] server responds 403 if request cannot be verified

Unit

Retry

• retry server errors: callback will retry on >=500 up to 3 times before dumping the last response
  • with expected backoff
  • including on timeout errors
• fail unrecoverable: callback retry will exit immediately on <500 errors

HMAC

• constructs valid signatures that can be parsed/verified as expected