[services] Move constants to a separate file in commtest

Summary:
ENG-1322
Moves constants from tools.rs to constants.rs.

Test Plan:
Check if it builds

cd services/commtest
cargo test --no-run

Reviewers: varun, jon, marcin, tomek

Reviewed By: jon, marcin, tomek

Subscribers: ashoat, tomek, atul, abosh

Differential Revision: https://phab.comm.dev/D5203