test-lib: add test for run_all_tests function

[zmiklank]

The test tests if testsuite ends with error if any ran test failed and if it ends with success if all tests pass.

[zmiklank]

[test-all]

[zmiklank]

Thanks for this great pull request. Would it be possible to add there also UNSTABLE tests?

I would rather do that in different testcase. Preferably also in other PR, when I find time to do it. But great idea, I will put it on my TODO list.

[zmiklank]

[test-all]