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/groups_controller_spec.rb

@@ -2870,8 +2870,6 @@ RSpec.describe GroupsController do
       end
 
       context "when rate limited" do
-        use_redis_snapshotting
-
         it "rate limits anon searches per user" do
           RateLimiter.enable