this removes the following warning:

```
DEPRECATION: `config.poll_interval = 0.5` will be removed in Sidekiq 4. Please update to `config.average_scheduled_poll_interval = 0.5`.
```

See #19498

setup gets called from the initializer, so it happens more than once in
a test run. Trying to drop the database again after the first process is
connected is.. ineffective. And entirely pointless.

Instead, defer creating the database to start_workers -- which only
happens once, right before we start doing anything real.

* Don't swallow output -- if there is any, it's probably useful
* Wait for the process to finish
* Use IPC instead of a sleep
* No need for a pidfile

Requiring sidekiq/testing changes stuff, so we need to counteract that
after we do so.

And given its potential to confuse things, let's do it up front, at a
predictable point.

Sidekiq::CLI#boot_system require "#{dummy_app_path}/config/environment.rb".
But this file has already been required in'test/support/integration/helper.rb'.
This patch will change to use Sidekiq::Launcher directly.

Use redis protocol

* .connect on a Redis connection wasn't valid
* Reset logger after we're done testing for redis connection to avoid
  "closed stream" error when starting server for real from a fork

In passing, avoid a blind retry in QC: instead, just fix the problem.