DEV: Respect SKIP_TEST_DATABASE when running rake db:create (#24407)

Why this change?

By default the `db:create` Rake task in activerecord creates the
databases for both the development and test environment. This while
seemingly odd is by design from Rails. In order to avoid creating the
test database, Rails supports the `SKIP_TEST_DATABASE` environment
variable which we should respect when creating the multisite test
database.

lib/tasks/db.rake

@@ -20,7 +20,8 @@ module MultisiteTestHelpers
   end
 
   def self.create_multisite?
-    (ENV["RAILS_ENV"] == "test" || !ENV["RAILS_ENV"]) && !ENV["RAILS_DB"] && !ENV["SKIP_MULTISITE"]
+    (ENV["RAILS_ENV"] == "test" || !ENV["RAILS_ENV"]) && !ENV["RAILS_DB"] &&
+      !ENV["SKIP_MULTISITE"] && !ENV["SKIP_TEST_DATABASE"]
   end
 end