Preivously, you could append your own hooks to parallel tests, but there
was no way to skip the Rails database parallelization hooks. There may
be cases where you want to implement your own database handling, so this
allows you to implement all of that in a `parallelize_setup` block
without having Rails create any databases for you.

To skip Rails database parallelization when running parallel tests you
can set it in the `parallelize` caller:

```ruby
parallelize(workers: 10, parallelize_databases: false)
```

Or in your application config:

```ruby
config.active_support.parallelize_test_databases = false
```

This is set to true by default.

Note: if you do not create databases for the processes, it is possible
for your test suite to deadlock. If you're seeing frequent deadlocks,
you may need to create more databases for the processes to use or adjust
your tests/code so it's less likely to cause database contention.