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.
In Go 1.16, the module-aware mode is enabled by default; to fall back to
previous behavior, the GO111MODULE environment variable should be set to
'auto'.
Details: https://golang.org/doc/go1.16
