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.
Common methods from applications/proto.py converted to the fixtures.
sysctl check moved to the specific file where it is using.
Some options moved to the constructor to have early access.
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.
Test case: "send a text message split into two fragments, then a continuation
frame with FIN = false where there is nothing to continue, then an unfragmented
text message, all sent in one chop".
The test case investigates immediate connection closing since there is no
message to continue.
The mirror server may send a response for the first frame before the test
сontinuation frame is received by the router. In this case, the test will
receive a text frame before the close frame.
