Most Python-based system tests need to know which ports were assigned to
a given test by bin/tests/system/get_ports.sh. This is currently
handled by inspecting the values of various environment variables (set
by bin/tests/system/run.sh) and passing the port numbers to Python
scripts via pytest fixtures. However, this glue code has so far been
copy-pasted into each system test using it, rather than reused.
Since pytest also looks for conftest.py files in parent directories,
move commonly used fixtures to bin/tests/system/conftest.py. Set the
scope of all the moved fixtures to "session" as their return values are
only based on environment variables, so there is no point in recreating
them for every test requesting them. Adjust test code accordingly.
gnutls-cli is tricky to script around as it immediately closes the
server connection when its standard input is closed. This prevents
simple shell-based I/O redirection from being used for capturing the DNS
response sent over a TLS connection and the workarounds for this issue
employ non-standard utilities like "timeout".
Instead of resorting to clever shell hacks, reimplement the relevant
check in Python. Exit immediately upon receiving a valid DNS response
or when gnutls-cli exits in order to decrease the test's run time.
Employ dnspython to avoid the need for storing DNS queries in binary
files and to improve test readability. Capture more diagnostic output
to facilitate troubleshooting. Use a pytest fixture instead of an
Autoconf macro to keep test requirements localized.
