Class usage came from the unittest framework and it was always redundant
after migration to the pytest. This commit removes classes from files
containing tests to make them more readable and understandable.
Prerequisites check moved to the module level to simplify class structure.
Discovery and prerequisites checks functions moved to the separate files.
Introduced "require" fixture to provide per-test requirements check.
To make fixtures accessible inside of setup methods in tests all these methods
are renamed to the "setup_method_fixture" and decorated by autouse flag.
Also all setup methods moved to the top of the files.
