Many software development projects struggle with creating and communicating a testing culture that is appropriate for the project's needs. This may degrade software quality by leaving defects undiscovered. Previous research suggests that social coding sites such as GitHub provide a collaborative environment with a high degree of social transparency. This makes developers' actions and interactions more visible and traceable. We conducted interviews with 33 active users of GitHub to investigate how the increased transparency found on GitHub influences developers' testing behaviors. Subsequently, we validated our findings with an online questionnaire that was answered by 569 members of GitHub. We found several strategies that software developers and managers can use to positively influence the testing behavior in their projects. However, project owners on GitHub may not be aware of them. We report on the challenges and risks caused by this and suggest guidelines for promoting a sustainable testing culture in software development projects.