This is a better suited question for StackOverflow but NUnit is a pretty standard and easy to use Unit Testing framework. You can follow this guide by Microsoft on Testing with NUnit.
Testing that the connection string for the server is valid and the server is online make for good Unit Tests, also doing some mock schema changes to your Source Database, running your SMO process, and testing the changes are replicated on the Target Database is a good Unit Test.
Ideally you should have a Unit Test for each piece of your database you want replicated from Source to Target, e.g. the Table schema, the data, indexes, constraints (primary keys, foreign keys), database properties and extended properties, etc.