[TwigBridge] fix  tests
minor #12142 [TwigBridge] fix `DumpExtension` tests (xabbuh)

This PR was squashed before being merged into the 2.6-dev branch (closes #12142).

Discussion
----------

[TwigBridge] fix `DumpExtension` tests

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        |

Make test compatible with the changes from #12125.

Commits
-------

e3332ad [TwigBridge] fix  tests
[TwigBundle] Fix that error page preview does not work for custom twig.exception_controller

The testErrorPageAction() always used the showAction() of TwigBundle's ExceptionController.
You can, however, configure an alternate controller by setting twig.exception_controller.

Thus, in order to get a proper preview, we need to forward to this configured controller (which
may be the default one).

This requires us to pass an additional parameter to ExceptionController::showAction to be able to
get the *error* page even if configured otherwise in the constructor.