Configure Capybara to use rack_test driver by default in system specs

[Nitemaeric]

Context

We want to configure Capybara to use the :rack_test driver by default and the :chrome_headless driver when js is required.

Changes proposed in this pull request

Add Capybara configuration.

Guidance to review

BAT reference can be found here: https://github.com/DFE-Digital/apply-for-teacher-training/blob/dd2212514ff076ddb6b423bdfac31615a7013406/spec/support/capybara.rb

Link to Trello card

https://trello.com/c/Bq7BJB6G/85-add-capybara-configuration?filter=label:Dev

Things to check

• If the code removes any existing feature flags, a data migration has also been added to delete the entry from the database
• This code does not rely on migrations in the same Pull Request
• If this code includes a migration adding or changing columns, it also backfills existing records for consistency
• If this code adds a column to the DB, decide whether it needs to be in analytics yml file or analytics blocklist
• API release notes have been updated if necessary
• If it adds a significant user-facing change, is it documented in the CHANGELOG?
• Required environment variables have been updated or added to the Azure KeyVault

Screenshots

[CatalinVoineag]

Pulled this on local, works as expected 👍🏻

[JamieCleare2525]

This will make some of my tests quick config change on my PR. But happy that this works.