Automatically maintain test database schema

* Move check from generated helper to test_help.rb, so that all
  applications can benefit
* Rather than just raising when the test schema has pending migrations,
  try to load in the schema and only raise if there are pending
  migrations afterwards
* Opt out of the check by setting
  config.active_record.maintain_test_schema = false
* Deprecate db:test:* tasks. The test helper is now fully responsible
  for maintaining the test schema, so we don't need rake tasks for this.
  This is also a speed improvement since we're no longer reloading the
  test database on every call to "rake test".
provide correct information [ci skip]
Use DATABASE_URL by default

See https://github.com/rails/rails/pull/13463#issuecomment-31480799 for full conversation.
Fixing nil conversion TypeError when using YAML coder on ActiveRecord::Store