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/presence_controller_spec.rb

@@ -45,8 +45,6 @@ RSpec.describe PresenceController do
 
   describe "#update" do
     context "in readonly mode" do
-      use_redis_snapshotting
-
       before { Discourse.enable_readonly_mode }
 
       it "produces 503" do