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.

首先，追蹤 tns test 指令及其基於 Karma 的單元測試設定。檢視 karma-sauce-launcher 如何納入現有的測試流程，並定義它所需的 flag 與 Sauce Labs 設定。完成的標準是：可以選取單元測試並在 Sauce Labs 裝置上執行，同時不影響本機裝置測試。