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/application_controller_spec.rb

@@ -932,8 +932,6 @@ RSpec.describe ApplicationController do
     context "with rate limits" do
       before { RateLimiter.enable }
 
-      use_redis_snapshotting
-
       it "serves a LimitExceeded error in the preferred locale" do
         SiteSetting.max_likes_per_day = 1
         post1 = Fabricate(:post)
@@ -1156,8 +1154,6 @@ RSpec.describe ApplicationController do
 
     before { RateLimiter.enable }
 
-    use_redis_snapshotting
-
     it "is included when API key is rate limited" do
       global_setting :max_admin_api_reqs_per_minute, 1
       api_key = ApiKey.create!(user_id: admin.id).key
@@ -1208,8 +1204,6 @@ RSpec.describe ApplicationController do
       SiteSetting.slow_down_crawler_user_agents = "badcrawler|problematiccrawler"
     end
 
-    use_redis_snapshotting
-
     it "are rate limited" do
       now = Time.zone.now
       freeze_time now