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/middleware/anonymous_cache_spec.rb

@@ -230,8 +230,6 @@ RSpec.describe Middleware::AnonymousCache do
   describe "#force_anonymous!" do
     before { RateLimiter.enable }
 
-    use_redis_snapshotting
-
     it "will revert to anonymous once we reach the limit" do
       is_anon = false