1. A non-transitory machine-readable storage medium comprising instructions executable by a processing resource to: instruct an external service virtualization system to simulate a fault of an external service in a simulation of the external service, wherein the simulation of the external service is accessed by an application during testing of the application; acquire a plurality of testing metrics representing a response of the application to the simulation of the fault of the external service during the testing of the application; generate a testing application fingerprint representing the response of the application to the simulation of the fault of the external service during the testing of the application, wherein the testing application fingerprint indicates, for each of the testing metrics, whether the testing metric became abnormal in response to the simulation of the fault; and associate the testing application fingerprint with the external service and the fault in a repository storing the testing application fingerprint.