Can both the test and live keys be put in the settings.php

I would encourage you to use environment variables to store different keys.

You could then do the following as an example in your settings.php:

$settings['stripe_webhooks_api_key'] = getenv('STRIPE_API_KEY');
$settings['stripe_webhooks_signing_secret_key'] = getenv('STRIPE_WEBHOOKS_SIGNING_SECRET');