Create mocks for better testing

[dharmit]

With devfile/library#62 going in, a bunch of unit tests broke in odo repository. These tests are being modified on odo repo for the time being instead of merging devfile/library#72 (since that seemed like a step backward), but it would be helpful to have mocks in place for better unit testing in tools dependent on the library.

This would be helpful because, at the moment, we need to call functions to create a valid DevfileData with different kinds of components and commands. Mocking these would provide more flexibility and cleaner unit tests.

cc @mik-dass @kadel @maysunfaisal