tests: Introduce iperf3 network performance tests

Add parametrized performance tests for each virtio-net backend
(passt, tap, gvproxy, vmnet-helper) in both upload and download
directions. Each test starts an iperf3 server on the host, runs
the iperf3 client inside a Fedora-based guest VM, and reports
throughput results as structured text/markdown via the Report
outcome.

Tests require IPERF_DURATION to be set at compile time and use a
podman-built rootfs with iperf3 pre-installed. They are skipped
when prerequisites are unavailable.

Signed-off-by: Matej Hrica <mhrica@redhat.com>

Author: mtjhrc