Increase unit test coverage of at_client_mobile package to > 80%

[gkc]

Describe the bug There are currently no unit tests of the at_client_mobile package 😞

Additional context https://stackoverflow.com/a/34698711 is a good summary of why this matters. Excerpt:

[Code coverage] has a clear relationship to how well-tested the code is, which in turn is our best way to increase confidence in its correctness. Code coverage is a measurable approximation of immeasurable qualities we care about.

[ksanty]

@gkc put you on this issue card. Please update if I got it wrong. :)

[gkc]

We'll be getting to this over time. Target for PR29 is > 25%. Once we hit that we'll set target > 40% and so on

[gkc]

less important than continuing to enhance functional and end-to-end test coverage