* Allow system tests using Rack::Test to run without Chrome

If you require "action_dispatch/system_test_case", the driven_by method
will be executed immediately.

* https://github.com/rails/rails/blob/v6.0.0/actionpack/lib/action_dispatch/system_test_case.rb#L162

Then the preload method is called in SystemTesting::Driver#initialize.

* https://github.com/rails/rails/blob/v6.0.0/actionpack/lib/action_dispatch/system_testing/driver.rb#L13
* https://github.com/rails/rails/blob/v6.0.0/actionpack/lib/action_dispatch/system_testing/browser.rb#L46-L63

Therefore, a "Webdrivers::BrowserNotFound" error occurs in the browser
preloading when you run system tests witout Chrome.

This commit avoid the error by lazy configuring the driver.

ref: #37410

* Don't preload the browser with :rack_test

If users specify `driven_by(:rack_test)`, it uses Chrome by default arguments.
However, `Rack::Test` does not use a browser and does not need to be preloaded.

Furthermore, it occurs `Webdrivers::BrowserNotFound` when run in
a container (or a machine) without Chrome.