Write simple unit tests for escrow to speed up testing of changes

We need to implement a testing framework. To start, being able to test the creating and paying of the hold invoices would be amazing.

If someone has a good background in testing, their help would be great appreciated. This is a good first issue and it doesnt require changing code , just adding a new framework for testing.

I have poorly implemented tests, so feel free to replace that entirely if its not fit for purpose.