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/serializers/topic_view_serializer_spec.rb

@@ -10,9 +10,6 @@ RSpec.describe TopicViewSerializer do
     JSON.parse(MultiJson.dump(serializer)).deep_symbolize_keys!
   end
 
-  # Ensure no suggested ids are cached cause that can muck up suggested
-  use_redis_snapshotting
-
   fab!(:topic)
   fab!(:user) { Fabricate(:user, refresh_auto_groups: true) }
   fab!(:user_2) { Fabricate(:user) }