We are an ISV partner and looking for ways to automate our tests for our package. We are looking for something like Mockito for Java and have found ApexMocks library, which is built on top of Stub API. However, it seems like we need to include the code in our package, which is a considerable disadvantage. On the other hand, I've heard people use Selenium for testing but as far as I know it is UI testing.
Our goal is to minimize manual testing for our package and make sure new changes do not break existing functionalities (proper unit tests and regression testing). Moreover, it would be nice to minimize our test run time and have our package upload faster. Looking for an optimal solution to solve this.