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/admin/backups_controller_spec.rb

@@ -156,8 +156,6 @@ RSpec.describe Admin::BackupsController do
       context "with rate limiting enabled" do
         before { RateLimiter.enable }
 
-        use_redis_snapshotting
-
         after { RateLimiter.disable }
 
         it "is rate limited" do