\Drupal\Core\Extension\Extension's constructor now validates input during development

Previously, \Drupal\Core\Extension\Extension accepted arbitrary input. This led to a lot of incorrect tests. assert()s have been added to ensure valid input.

To avoid any backwards compatibility breaks, assert() is used instead of throwing exceptions. This means that no production site can possibly be negatively impacted, only development environments could be.

In Drupal core, only test coverage is constructing \Drupal\Core\Extension\Extension objects, and the same should be true in contrib and custom code. Tests constructing invalid Extension objects may need to be updated.