[Commtest] fix Clippy warnings and typos

Summary:
As suggested by @bartek I started using clippy linter for rust, but it's hard when it generates a lot of warnings in old code - fixing all, and some typos.

Depends on D9310

Test Plan: Run tests

Reviewers: michal, bartek, varun, jon

Reviewed By: michal, bartek

Subscribers: ashoat, tomek, bartek

Differential Revision: https://phab.comm.dev/D9311