Fix test failures stemming from Dynamic Page Cache

So, I don't know smart cache at all, but could you do something like this:

Cache::invalidateTags(['page_manager_page:' . $entity->id()]);

... except with (a) the right API and what-not (since you're not being passed the entity object) and (b) making sure we actually have that cache tag set somewhere (which I don't think we do right now).

+++ b/src/EventSubscriber/RouteNameResponseSubscriber.php
@@ -0,0 +1,61 @@
+      $cacheability_metadata->addCacheTags(['route_name:' . $this->routeMatch->getRouteName()]);

This is a very generic cache tag. I'd say it should be page_manager_route_name:$route_name or something like that, to prevent a conflict in case core ever does something like that.

Otherwise looks great :)

@Wim Leers: Tim and I discussed on IRC last night that this is something Views might need as well (Tim's going to do some research/testing today) so it very well might make sense for core to do this.

Although, would this really be a "conflict"? Isn't adding a cache tag that's already there just a no-op? If core later starts adding this same cache tag, then the subscriber could be removed and no other code would need to be change, which might actually be a good thing. :-)

Although, would this really be a "conflict"? Isn't adding a cache tag that's already there just a no-op? If core later starts adding this same cache tag, then the subscriber could be removed and no other code would need to be change, which might actually be a good thing. :-)

Hah! Genius! :)