Add guidance for test transactions

[yashrajd]

Describe your content request Content & designs for test transaction features in wallets

Rationale

• test transactions are a common usage pattern in bitcoin as well as fiat transactions
• this was recently discussed in a podcast episode around 22 mins
• test transactions are great way silent payments users to validate and feel comfortable about sending bitcoin to a different address than that provided by the receiver

Suggestions @rabbitholiness and max pain truly created prototypes (here and here), which could be used to craft content for the issue.

Happy to work on this... actively taking suggestions for where in the guide this content should go.