[services] Clean up test_all_services.sh

Summary: Relevant Linear issue here. Used $(...) syntax instead of legacy backticks ...

Test Plan: ShellCheck, close reading

Reviewers: ashoat, atul, palys-swm

Reviewed By: ashoat

Subscribers: Adrian, karol-bisztyga

Differential Revision: https://phab.comm.dev/D4451