test(s2n-quic-dc): add tests for map events

[camshaft]

Description of changes:

This change adds several snapshot tests to the map events.

Call-outs:

In order to actually implement this I had to:

• Add support for redacting fields in a snapshot, as they are random values
• implementing Subscriber for Arc<T> where T: Subscriber
• Add a testing method that generates a pair of secrets given 2 maps, since they can't actually send packets to themselves since secrets are unidirectional

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.