FIX: ensure we consistently pick the same topic for bench

We pick the first topic with 30 responses as our bench topic.

Previously we simply picked the last topic, but hand no guarantee on ordering.

This also attempts to correct previous runs of the bench.

script/profile_db_generator.rb

@@ -65,9 +65,12 @@ def create_admin(seq)
 end
 
 def ensure_perf_test_topic_has_right_title!
-  t = Topic.where(archetype: :regular).last
-  t.title = "I am a topic used for perf tests"
-  t.save! if t.title_changed?
+  title = "I am a topic used for perf tests"
+  # in case we have an old run and picked the wrong topic
+  Topic.where(title: title).update_all(title: "Test topic #{SecureRandom.hex}")
+  t = Topic.where(archetype: :regular, posts_count: 30).order(id: :desc).first
+  t.title = title
+  t.save!
 end
 
 # by default, Discourse has a "system" and `discobot` account