tests: add send coins test

[bznein]

Note: based on #3666

This is a test that:

• sets up regtest
• send coints to RBTC account 1
• creates a second RBTC account
• send coins from 1 to 2
• Checks that transactions appear in the list

Before asking for reviews, here is a check list of the most common things you might need to consider:

• updating the Changelog
• writing unit tests
• checking if your changes affect other coins or tokens in unintended ways
• testing on multiple environments (Qt, Android, ...)
• having an AI review your changes