floss: Add unittest for get_hci_index_from_pid_path and fix warnings

Warnings:
- rustfmt_bindings is deprecated
- redundant clone to a reference

Bug: 356808668
Tag: #floss
Test: mmm packages/modules/Bluetooth
Test: ./floss/build/build-in-container.py
Flag: EXEMPT, Floss-only changes
Change-Id: Ib0e87446b5b045fdaf77ead5b47044504a46dffe