DEV: Call Discourse.redis.flushdb after the end of each test (#29117)

There have been too many flaky tests as a result of leaking state in
Redis so it is easier to resolve them by ensuring we flush Redis'
database.

Locally on my machine, calling `Discourse.redis.flushdb` takes around
0.1ms which means this change will have very little impact on test
runtimes.

spec/lib/topic_query_spec.rb

@@ -1489,8 +1489,6 @@ RSpec.describe TopicQuery do
   end
 
   describe "#list_suggested_for" do
-    use_redis_snapshotting
-
     def clear_cache!
       Discourse.redis.keys("random_topic_cache*").each { |k| Discourse.redis.del k }
     end