Add mock for `getSystemTemperatureUnit` (`UpsServiceClient `)

[Ché Armstrong]

Describe the solution you'd like
A way to mock getSystemTemperatureUnit (UpsServiceClient ) via the virtual-alexa library.

Describe alternatives you've considered
Using nock to capture this call (outside of virtual-alexa). But having it it as part of the library would make testing and test setup better/easier for each test case.

Additional context
API ref - https://ask-sdk-for-nodejs.readthedocs.io/en/latest/Calling-Alexa-Service-APIs.html#upsserviceclient