The included function tests for validating each meta tag plugin works correctly were rewritten for Metatag 2.0.0, and each submodule's test class was updated to match the new structure.
Old structure
The tags tests started with \Drupal\Tests\metatag\Functional\MetatagTagsTestBase. Each module that provided meta tag plugins extended this class. Each child class included a variable that defined which meta tags to test. Each meta tag is tested individually using a @dataProvider on MetatagTagsTestBase::testTagsArePresent() and MetatagTagsTestBase::testTagsInputOutput(), a series of methods on the child class could be added to customize the test workflow, following a somewhat "magic" naming convention.
New structure
The tags tests start with \Drupal\Tests\metatag\Functional\TagsTestBase. Each module that provided meta tag plugins extends this class. Every meta tag plugin available at the time is tested at once in testTags(). Each meta tag plugin overrides variables and attribute from MetaNameBase to control the tests, so all of the test logic is encompassed in the meta tag plugin itself.
