Tests - Dynamically generate self-signed certificates

[sjtrny]

Feature Proposed

The keys and certificates in examples/tls are used by the test cases to verify TLS behaviour. These do eventually expire and are really a build artefact, not something that we should be storing in the repository.

I propose that they are dynamically generated as part of the test process and then removed upon completion. There should be an optional flag to retain them after tests are run so that we can inspect their contents for debugging purposes.