After the launch of the project, the testing infrastructure was shared with
nginx project in some cases. To avoid port overlap, a decision was made
to shift the port range for Unit tests. This problem was resolved a long time
ago and is no longer relevant, so it is now safe to use port 8XXX range as the
default, as it is more appropriate for testing purposes.
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.
Previously, it was necessary to support older versions of Python for
compatibility. F-strings were released in Python 3.6. Python 3.5 was
marked as unsupported by the end of 2020, so now it's possible to start
using f-strings safely for better readability and performance.
