Improve integration tests coverage

Original Author: @slonokot

Follow up issue on the initial integration tests

Our exec and test-remote cmds aren't checking properly the outcomes with the existing mocks.

* Evaluate gomock framework
* Do a research on other solutions (say terraform)
* validate parallelism on tests work at the moment they are running sequentially
* do more "typed" error tests validations: we currently check error messages equality instead of checking error types and values where needed

Start by reading the existing integration tests and mocks used by the exec and test-remote commands. Evaluate gomock and the mentioned Terraform approach, then check whether the tests can run in parallel and where typed error validation is needed. Done means outcomes are properly verified, parallelism is validated, and error types and values are checked where appropriate.

Written by the indexing model from the issue text.