Validate test metadata

[vashirov]

Issue Description
Metadata in our tests' docstrings is used for test case and requirements import in our test case management system. But it's prone to copy-paste issues (duplicate unique test case IDs), formatting issues (RST parser is not always happy) or it's simply missing. We should use automatic validation for test metadata as part of our PR CI so that all new test cases have valid docstrings and unique IDs.