Support running unit tests in Sauce Labs

Is your feature request related to a problem? Please describe.
Development of mobile application requires testing the application on different API Levels (OS Versions) and devices. It will be great in case this testing can be based on cloud providers that allow running the application on real devices.

Describe the solution you'd like
Currently the unit tests are run only on attached devices, but it will be great in case it is possible to run them in cloud provider, like Sauce Labs. I imagine having a flag to tns test command that specify the tests should be run in Sauce Labs.

Describe alternatives you've considered
Integration of other cloud providers, even those who allow using emulator - Appetize for example.

Additional context
As CLI uses karma to run the unit tests, it could benefit of using karma-sauce-launcher.

Empieza siguiendo el comando tns test y su configuración de pruebas unitarias basada en Karma. Revisa cómo encajaría karma-sauce-launcher en el flujo de pruebas existente y define el flag y la configuración de Sauce Labs que necesita. Se considera terminado cuando se puedan seleccionar y ejecutar pruebas unitarias en dispositivos de Sauce Labs sin afectar a las pruebas en dispositivos locales.