Release testing instructions for WC Payments 3.9.0

Add IPP print receipt preview to Card Readers page

Expected outcome

There is a Preview a printed receipt link under the Receipt Details tab of the Card Readers page of the Payments menu. By clicking this link, a merchant can see a preview of a printed receipt.

Testing instructions

1. Enable Card Readers in admin menu by following instructions in this PR
2. Go to Payments -> Card Readers -> Receipt details
3. Modify any of available settings. Note that logo is not available to be previewed yet.
4. Click on Preview a printed receipt link
5. Observe that the generated receipt contains the modified settings data
6. Reload the page
7. Observe that the generated receipt contains the original unmodified data