Add mocks and UTs for docker model status

[doringeman]

$ make mock && make unit-tests
Generating mocks...
Mocks generated!
Running unit tests...
?   	github.com/docker/model-cli	[no test files]
?   	github.com/docker/model-cli/desktop	[no test files]
?   	github.com/docker/model-cli/mocks	[no test files]
=== RUN   TestStatus
=== RUN   TestStatus/running
=== RUN   TestStatus/not_running
=== RUN   TestStatus/request_with_error
=== RUN   TestStatus/failed_request
--- PASS: TestStatus (0.00s)
    --- PASS: TestStatus/running (0.00s)
    --- PASS: TestStatus/not_running (0.00s)
    --- PASS: TestStatus/request_with_error (0.00s)
    --- PASS: TestStatus/failed_request (0.00s)
PASS
ok  	github.com/docker/model-cli/commands	0.727s
Unit tests completed!

[ilopezluna]

Thanks! 👏

[ilopezluna]

why cmd.OutOrStdout() instead of os.Stdout? Also why to extract os.Exit into a variable?
I assume it has to do with being able to assert the output in the test but I'm unsure of how it works.

[doringeman]

So I can capture it in the UT where I overwrite cmd's output.
https://github.com/docker/model-cli/pull/20/files/d706eeb6e5433e561bc3e661066ef3a1cfe07bd4