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.
While alternating between running priv and unpriv tests
locally, it happens that unpriv tests can't bind mount
or create sub directories inside directories created
by root. This patch fixes this by pointing "rootfs"
to temporary directory.
Now the priv and unpriv test uses the same test function.
