[CI OAI-FlexRIC] Improve CI pipeline

1. Run the iperf instead of ping
2. SIGTERM added for nearRT-RIC so it shall gracefully stop in CI; modify the line for nearRT-RIC service analysis

TODO:

• change the xml test cases and put one CI dedicated xApp (once created)
• update the submodule once mosaic5g/flexric!84 (merged) is merged

Note: CI dedicated xApp dropped due to time limitations