Use Pebble for tests.

[buchdag]

The PR replace Boulder with Pebble where possible (the only test that require Boulder for now is ocsp_must_staple).

Pebble is specifically made for tests, way easier and faster to setup (~5s on GitHub Actions vs ~100s with Boulder).

Quoting Pebble's doc:

Pebble has several top level goals:

1. Provide a simplified ACME testing front end
2. Provide a test-bed for new and compatibility breaking ACME features
3. Encourage ACME client best-practices
4. Aggressively build in guardrails against non-testing usage

Using Pebble also opens up testing on arm/v7 and arm64 archs, as it is relatively easy to build on those archs.

Local tests can now be run against either Pebble or Boulder, similar to the choice between 2 and 3 containers setups.