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.

lib/redis_snapshot.rb

@@ -1,34 +0,0 @@
-# frozen_string_literal: true
-
-class RedisSnapshot
-  def self.begin_faux_transaction(redis = Discourse.redis)
-    @stack ||= []
-    @stack.push(RedisSnapshot.load(redis))
-  end
-
-  def self.end_faux_transaction(redis = Discourse.redis)
-    @stack.pop.restore(redis)
-  end
-
-  def self.load(redis = Discourse.redis)
-    keys = redis.keys
-
-    values = redis.pipelined { |batch| keys.each { |key| batch.dump(key) } }
-
-    new(keys.zip(values))
-  end
-
-  def initialize(dump)
-    @dump = dump
-  end
-
-  def restore(redis = Discourse.redis)
-    redis.pipelined do |batch|
-      batch.flushdb
-
-      @dump.each { |key, value| batch.restore(key, 0, value) }
-    end
-
-    nil
-  end
-end