Add BlockVariantInterface (and Trait), and BlockPluginCollection from Page Manager

Just noting that I am working on this at Drupalcorn.

I took a crack at this but I know nothing about testing and am hung up there. I moved the two tests that seemed likely but could not figure out how to run the tests. If I do --file [testfile] it comes back with "ERROR: No valid tests were specified.".

Unassigning this from myself because I'm heading home from Drupalcorn and honestly don't see me being able to get back to this in the next 2 weeks. Hope it helps some. :)

Working on this at MWDS

I moved some the tests from /src/Tests/src/Unit to /tests/src/Unit. They should execute and pass now.

+++ b/src/Plugin/ConditionVariantInterface.php
@@ -0,0 +1,92 @@
+interface ConditionVariantInterface extends VariantInterface {

+++ b/src/Plugin/ConditionVariantTrait.php
@@ -0,0 +1,108 @@
+trait ConditionVariantTrait {

Both of these seem completely unnecessary to me. We're moving Page Manager variants to a config entity and those will always have selection criteria handling at the entity level and don't need this abstraction. Remove these from the patch and the rest looks pretty good to me.

+++ b/src/Plugin/BlockVariantTrait.php
@@ -0,0 +1,120 @@
+  public function getRegionNames() {
+    return [
+      'top' => 'Top',
+      'bottom' => 'Bottom',
+    ];
+  }

I really don't like that we're implementing this in a trait. I'd much rather see us provide a BlockVariantInterface implementing class that provides these regions instead of having them on the trait.

So, this feels like two issues to me. Can we break apart the EntityView block & deriver from the BlockVariantInterface & Plugin collection and stuff? One of these is totally applicable to core's uses and the others are support for panels. I think it'll make review easier, I made it to the deriver class and was like "wait am I still reviewing the entity view block patch?" and indeed I was.

I think both of these can probably go in, but I have no doubt about the block and the deriver and I have some additional review and effort I want to see on the variant stuff, so lets simplify a bit and get some of this committed now.

Eclipse

Split the EntityView block into another issue. #2568741: Add EntityView block from Page Manager

Patch is the same but with the Entity View block and deriver removed.

Setting back to needs review so we can begin sorting out the block view variant.

Added #2573413: Fix wizard tests to address the failing Wizard tests that don't seem to have anything to do with this patch.

Marking as a duplicate to the fixed #2511566: Add an abstract version of Drupal\page_manager\Plugin\DisplayVariant\BlockDisplayVariant from Page Manager issue.