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/users_controller_spec.rb

@@ -380,8 +380,6 @@ RSpec.describe UsersController do
       context "with rate limiting" do
         before { RateLimiter.enable }
 
-        use_redis_snapshotting
-
         it "rate limits reset passwords" do
           freeze_time
 
@@ -4410,8 +4408,6 @@ RSpec.describe UsersController do
     end
 
     context "with a session variable" do
-      use_redis_snapshotting
-
       it "raises an error with an invalid session value" do
         post_user
 
@@ -5625,8 +5621,6 @@ RSpec.describe UsersController do
   describe "#enable_second_factor_totp" do
     before { sign_in(user1) }
 
-    use_redis_snapshotting
-
     def create_totp
       stub_secure_session_confirmed
       post "/users/create_second_factor_totp.json"