MDL-67969 lib: Fix duplicate blockinstanceid for fake blocks

lib/outputrenderers.php

@@ -1745,7 +1745,7 @@ class core_renderer extends renderer_base {
         $id = !empty($bc->attributes['id']) ? $bc->attributes['id'] : uniqid('block-');
         $context = new stdClass();
         $context->skipid = $bc->skipid;
-        $context->blockinstanceid = $bc->blockinstanceid;
+        $context->blockinstanceid = $bc->blockinstanceid ?: uniqid('fakeid-');
         $context->dockable = $bc->dockable;
         $context->id = $id;
         $context->hidden = $bc->collapsible == block_contents::HIDDEN;

lib/templates/block.mustache

@@ -1,5 +1,5 @@
 {{!
-    @template theme_boost/block
+    @template core/block
 
     Example context (json):
     {