Read-Only Container is not working properly.

Found while working on #2497243: Replace Symfony container with a Drupal one, stored in cache:

Problem/Motivation

- The solution to using PhpStorage was that production systems can use the ReadOnlyStorage.

That does not work in practice however, because the test that was supposed to test that itself has a bug.

     // Instantiate it a second time and we should still get a ContainerBuilder
     // class because we are using the read-only PHP storage.
-    $kernel = $this->getTestKernel($request, $modules_enabled, TRUE);
+    $kernel = $this->getTestKernel($request, NULL, TRUE);

is needed to fix the test and then it fails.

The reason this worked was, because if $modules_enabled is passed, then updateModules() is called before boot(), which obviously would not happen on a production system.

Proposed resolution

- Implement #2497243: Replace Symfony container with a Drupal one, stored in cache to make us independent of PHPStorage as needed storage.

Other possibilities might work and will need to be implemented if #2497243: Replace Symfony container with a Drupal one, stored in cache is not done.

Remaining tasks

- Fix the test
- Wait for #2497243: Replace Symfony container with a Drupal one, stored in cache

User interface changes

- None

API changes

- None

Data model changes

- None