specs: temporarily use memory queue as a default

In an effort to stabilise our specs and reduce the amount of overflow from
each spec to the next, this patch temporarily sets the default queue type
to be the memory queue.

Fixes elastic#9601

spec/spec_helper.rb

@@ -44,9 +44,7 @@ def puts(payload)
       # Some tests mess with the settings. This ensures one test cannot pollute another
       LogStash::SETTINGS.reset
 
-      LogStash::SETTINGS.set("queue.type", "persisted")
-      LogStash::SETTINGS.set("queue.page_capacity", 1024 * 1024)
-      LogStash::SETTINGS.set("queue.max_events", 250)
+      LogStash::SETTINGS.set("queue.type", "memory")
       LogStash::SETTINGS.set("path.data", temp_directory)
 
       example.run