Mobile apps are born to work in an environment with ever-changing network connectivity, random hardware interruption, unanticipated task switches, etc. However, such interference cases are often oblivious in traditional mobile testing but happen frequently and sophisticatedly in the field, causing various robustness, responsiveness and consistency problems. In this paper, we propose JazzDroid to introduce interference to mobile testing. JazzDroid adopts a gray-box approach to instrument apps at binary level such that interference logic is inlined with app execution and can be triggered to effectively affect normal execution. Then, JazzDroid repeatedly orchestrates the instrumented app through app developers' existing tests and continuously randomizes interference on the fly to reveal possible faulty executions. Upon discovering problems, JazzDroid generates a test script with the user inputs from developers' tests and the interference injected for developers to reproduce the problems. At a high level, JazzDroid can be seamlessly integrated into app developers' testing procedures, detecting more problems from existing tests. We implement JazzDroid to function on unmodified apps directly from app markets and interface with de facto industrial testing toolchain. JazzDroid improves mobile testing by discovering 6x more problems, including crashes, functional bugs, UI consistency issues and common bug patterns that fail numerous apps.