DEV: Add tests to S3Helper.ensure_cors and move rules to class (#14767)

In preparation for adding automatic CORS rules creation
for direct S3 uploads, I am adding tests here and moving the
CORS rule definitions into a dedicated class so they are all
in the one place.

There is a problem with ensure_cors! as well -- if there is
already a CORS rule defined (presumably the asset one) then
we do nothing and do not apply the new rule. This means that
the S3BackupStore.ensure_cors method does nothing right now
if the assets rule is already defined, and it will mean the
same for any direct S3 upload rules I add for uppy. We need
to be able to add more rules, not just one.

This is not a problem on our hosting because we define the
rules at an infra level.

lib/s3_helper.rb

@@ -140,12 +140,7 @@ class S3Helper
     end
 
     unless rule
-      rules = [{
-        allowed_headers: ["Authorization"],
-        allowed_methods: ["GET", "HEAD"],
-        allowed_origins: ["*"],
-        max_age_seconds: 3000
-      }] if rules.nil?
+      rules = [S3CorsRulesets::ASSETS] if rules.nil?
 
       s3_resource.client.put_bucket_cors(
         bucket: @s3_bucket_name,