Improve test coverage for the devworkspace-operator

[ibuziuk]

The goal is to sufficiently improve the test coverage of the devworkspace-operator and enable codecov checks in the repo. As an example we could use the same approach as in the https://github.com/eclipse-che/che-operator

[AObuchow]

@amisevsk @ibuziuk I think we can close this issue now, as #929, #930 and #996 were created and resolved.