Partial fix of database url tests

Prior to #13463 when `DATABASE_URL` was set, Rails automagically used that value instead of the database.yml. There are tests in dbs_test that expect this to still be true. After that PR, `RAILS_DATABASE_URL` is expected to be read into the YAML file via ERB, this PR fixes that behavior.

Note: this does not entirely fix the tests. It seems that `ActiveRecord::Tasks::DatabaseTasks.current_config` does not process the url string correctly (convert it into a hash), and ` ActiveRecord::Tasks::DatabaseTasks.structure_load(current_config, filename)` as well as other methods in `DatabaseTasks` expect a hash.

It seems like we should involve the resolver somewhere in this process to correctly convert the database url, I do not know the best place for that /cc @josevalim

railties/test/application/rake/dbs_test.rb

@@ -20,9 +20,12 @@ def database_url_db_name
       end
 
       def set_database_url
-        ENV['DATABASE_URL'] = File.join("sqlite3://:@localhost", database_url_db_name)
+        ENV['RAILS_DATABASE_URL'] = File.join("sqlite3://:@localhost", database_url_db_name)
         # ensure it's using the DATABASE_URL
         FileUtils.rm_rf("#{app_path}/config/database.yml")
+        File.open("#{app_path}/config/database.yml", 'w') do |f|
+          f << {ENV['RAILS_ENV'] => '<%= ENV["RAILS_DATABASE_URL"] %>'}.to_yaml
+        end
       end
 
       def expected