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/invites_controller_spec.rb

@@ -771,8 +771,6 @@ RSpec.describe InvitesController do
       describe "rate limiting" do
         before { RateLimiter.enable }
 
-        use_redis_snapshotting
-
         it "can send invite email" do
           sign_in(user)
 
@@ -1598,8 +1596,6 @@ RSpec.describe InvitesController do
       RateLimiter.enable
     end
 
-    use_redis_snapshotting
-
     it "resends all non-redeemed invites by a user" do
       freeze_time