Also fixed various pylint errors and style issues.

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.

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.

This change is necessary to separate the logic
and prevent possible circular dependency.

This patch also enables multiversion tests running for Java.

Previously, for PHP, Ruby, and Perl the latest version was always loaded
in multi-version tests.