do not trigger AR lazy load hook before initializers ran.

[Rafael Mendonça França & Yves Senn]

This require caused the `active_record.set_configs` initializer to
run immediately, before `config/initializers`. This means that setting any
configuration on `Rails.application.config.active_record` inside of
an initializer had no effects when rails was loaded through `rake`.

Introduced by #6518

/cc @rafaelfranca
bring back `db:test:prepare`.

This reverts deprecations added in #13528.
The task is brought back for two reasons:
  1. Give plugins a way to hook into the test database initialization process
  2. Give the user a way to force a test database synchronization

While `test:prepare` is still a dependency of every test task, `db:test:prepare`
no longer hooks into it. This means that `test:prepare` runs before the schema
is synchronized. Plugins, which insert data can now hook into `db:test:prepare`.

The automatic schema maintenance can't detect when a migration is rolled-back,
modified and reapplied. In this case the user has to fall back to `db:test:prepare`
to force the synchronization to happen.
Merge pull request #17739 from rails/bring_back_db_test_prepare

bring back `db:test:prepare`.