Part of #3196

When a user attempts to connect a fresh site to Stripe. We currently allow them to connect to a live account via the Create or connect an account link on the Stripe OAuth connect UI.

If you want to connect to a test account, you currently have to connect to a live account and then connect again via the edit account keys page. We should streamline the process for folks setting up a test/staging site so they can connect to a test account. This is similar to what WooPayments does on their connect page:

^{WooPayments connect in staging mode option}

REQUIREMENTS

• Update the WC_Stripe_Connect_API class to support test mode requests #3291