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.
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.
A race may occur between the router process restart and the main process
sending a notification to the running controller. For example, a test script
detects the new process and starts performing a smoke test, but the controller
has not yet received the 'remove PID' notification, so the connection to the
router is broken and any attempt to update the configuration will cause an
error.
The solution is to perform several attempts to reconfigure Unit with a short
delay between failures.
Running `test_respawn_` test cases on a machine with Unit
daemon in background would fail tests because `ps ax` was
used without filtering out other unit instances.
This patch also prevents from tests killing other Unit
processes not related to tests.
