CI: refactor iperf3 UDP analysis function
- add helper function to covert reported bitrate values to Mbps for calculations
- clean up the UDP analysis function, improve test report formatting
- fix an issue where Gbps values were incorrectly reported (example: https://jenkins-oai.eurecom.fr/job/RAN-SA-AERIAL-CN5G/4802/)
- update CI unit test report checks to match the new formatting