[meta] Expand phpunit tests for \Drupal\Component\Plugin classes

Task to add additional test coverage for the various classes in \Drupal\Component\Plugin. Also, the current tests are in the core directory, which may mean they are intertwined with core overrides. These should be split into separate tests if that is the case.

See #1938068: Convert UnitTestBase to PHPUnit → .

Current issues:

• #2368585: Expand unit testing for Drupal\Component\Plugin\ContextAwarePluginBase →
• #2370183: Expand unit testing for Drupal\Component\Plugin\Discovery\DiscoveryTrait →
• #2371531: Expand unit testing for Drupal\Component\Plugin\PluginManagerBase →
• #2375737: Expand unit testing for Drupal\Component\Plugin\Factory\ReflectionFactory →
• #2379419: Expand unit testing for Drupal\Component\Plugin\Discovery\StaticDiscoveryDecorator →
• 🐛 Drupal\Component\Plugin\Derivative\DeriverBase is designed to error out. Needs work

Drupal\Component\Plugin\Context\Context gets its own section, due to needing a little refactoring and bug-fixing.

• #2378311: Expand unit testing for Drupal\Component\Plugin\Context\Context →
• 📌 Modify Drupal\Component\Plugin\Context\ContextInterface to allow for unit testing. Postponed: needs info
• 🐛 Inconsistent use of ContextDefinitionInterface in Drupal\Component\Plugin\Context\Context::getConstraints() Needs work