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/requests/topics_controller_spec.rb

@@ -2249,8 +2249,6 @@ RSpec.describe TopicsController do
   end
 
   describe "#show" do
-    use_redis_snapshotting
-
     fab!(:private_topic) { pm }
     fab!(:topic) { Fabricate(:post, user: post_author1).topic }