Incorrect line item price sent to Afterpay for quantities > 1

RTBC!

Thanks @corniflex!

Rebased against 1.x and updated changelog.

I don't have access to a site with Afterpay anymore so wasn't able to run the API integration test. (It's not extensive anyway, but much better than nothing.) I've updated the test so that instead of adding one item to the cart, it adds two items which now cost half the price. Otherwise the test remains unchanged, so hopefully it'll work - but as I say, I can't run it without sandbox creds.

If some kind soul with access to a sandbox wants to run the tests, I'd very much appreciate it.

There's a functional javascript test that can be run to test the checkout and
payment capture/void process. The test assumes that Afterpay will accept a total
of ~45 USD.

1. Set up an account on the sandbox portal (eg. at
   https://portal.sandbox.afterpay.com/us). You'll need to set the password and
   it should already have a single saved payment method.
2. Set the following environment variables:
   • COMMERCE_AFTERPAY_TEST_MERCHANT_ID<\code>
   • COMMERCE_AFTERPAY_TEST_SECRET_KEY<\code>
   • COMMERCE_AFTERPAY_TEST_COUNTRY_CODE<\code>
   • COMMERCE_AFTERPAY_TEST_CUSTOMER_EMAIL<\code>
   • COMMERCE_AFTERPAY_TEST_CUSTOMER_PASSWORD<\code>

https://git.drupalcode.org/project/commerce_afterpay#testing-against-the-sandbox-api

Merging because it makes sense per the documentation and is clearly major, but leaving open for feedback on \Drupal\Tests\commerce_afterpay\FunctionalJavascript\ApiIntegrationTest.