[TESTS] Implement Mocking where needed

[paulotruta]

Using mocking to be able to test some of the functions in modules that depend on dynamic functionality (measuring available disks and disk sizes, getting a list of running apps, etc).

This is an iteration on our efforts to provide complete app testing.

There are two types of mocking we will be interested in doing:

• function return mocking, which mocks the return value of a function, be it internally or in external dependencies / apis. This can be achieve through clever implementation of Interfaces in Golang. More information about Interfaces here, and a tutorial on mocking using Interfaces here.
• DB call mocking, that we can leverage to simulate the existence of various states within the application. [MORE INFORMATION AND REFERENCES NEEDED HERE]

Some task guidelines for a PR:

• Implement interfaces for mocking data that is called within storage.getDevicesSpaceUsage and storage.getDevices.
• Reliably test all branches of those functions
• Gather information about what in the codebase can benefit from this approach
• Research DB call mocking to test edgeapps.go