The test inherits from two different test fixtures, ServiceContextMongoDTest and CatalogCacheTestFixture. During setup of the first, the JournalFlusher background thread is started, which will asynchronously create an operation context, which takes a raw pointer to the service context's tick source and shortly after calls getTicks() on it. During setup of the second, the tick source of the global service context is set to a mock tick source, which triggers destruction of the original tick source and can lead the JournalFlusher to access freed memory.