public class MyReportService {
    DataSource source;
    public MyReportService(DataSource ds) {
        this.source = ds;
    }

   // Bunch of other methods that do things with with data source.
}

DataSource myMockDataSource = generateMyMockDataSourceSomehow();
MyReportService service = new MyReportService(myMockDataSource);

Test.startTest();
Integer retVal = service.answerMagicalQuestion();
Test.stopTest();

System.assertEquals(42, retVal, 'magical answer wrong');