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，跟踪 tns test 命令及其基于 Karma 的单元测试配置。检查 karma-sauce-launcher 如何适配现有的测试流程，并定义它所需的 flag 和 Sauce Labs 配置。完成的标准是：可以选择单元测试并在 Sauce Labs 设备上运行，同时不影响本地设备测试。