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/models/invite_spec.rb

@@ -221,8 +221,6 @@ RSpec.describe Invite do
           3.times { Invite.generate(user, email: "test@example.com") }
         end
 
-        use_redis_snapshotting
-
         it "raises an error" do
           expect { Invite.generate(user, email: "test@example.com") }.to raise_error(
             RateLimiter::LimitExceeded,