Initial unit tests outline.
Migrated base test structure from 7.x-2.x branch.
Migrated list subscription test from 7.x-2.x branch.
Removed old test code, refactored lists test class.
Populated subscribe / unsubscribe test methods.
Added MailChimp campaigns test class, added test class constructors.
Added missing construct params to test classes.
Minor test fixes, added list retrieval test.
Added test case for multi-list retrieval.
Added unit tests; placeholders for additional unit tests.
Additional unit tests, data structure fixes.
Added list segment tests. Fixed a few failing test cases.
Added list segment tests, updated tests for clarity.
Added webhook tests.
Added campaign tests.
Populated missing webhook test methods.