tests: Add robot to CI - #352

Options where to place new Loadtest workflow:

1. Inside a test.yml, just after all go and python tests (imho, bad choice for plenty of reasons)
2. As an additional job inside test.yml with defined "needs" parameter, which will run only if first job (go and python tests) succeeded.
3. As a separate workflow with trigger "workflow_run", so it will run only if workflow test.yml succeded. Best choice, I suppose, but the problem is that "workflow_run" trigger only works on default branch (main) pushes.
4. As a separate workflow, which will run in parallel with all other tests.

Well, it is possible to combine (3) and (4) options:

• run loadtest after other workflows on main branch
• run loadtest in parallel on all other branches / PRs

5. Make both test.yml and loadtest.yml workflows reusable. Create generic workflow, which will call them in right order.