Source Mailchimp: enable high test strictness level in SAT

[alafanechere]

What

A test_strictness_level field was introduced to Source Acceptance Tests (SAT).
Mailchimp is a generally_available connector, we want it to have a high test strictness level.

This will help:

• maximize the SAT coverage on this connector.
• document its potential weaknesses in term of test coverage.

How

1. Migrate the existing acceptance-test-config.yml file to the latest configuration format. (See instructions here)
2. Enable high test strictness level in acceptance-test-config.yml. (See instructions here)
3. Commit changes on acceptance-test-config.yml and open a PR.
4. Run SAT with the /test command on the branch.
5. If tests are failing please fix the failing test or use bypass_reason fields to explain why a specific test can't be run.

[davydov-d]

Implemented here