[services] Use [[ ]] instead of [ ] in test_service.sh

Summary: This is the final diff needed to make brackets consistent in bash scripts.

Test Plan:
@ashoat's close reading and this git grep command:

The command may be confusing to parse, so I plugged it into Regex101:

Reviewers: ashoat, atul, palys-swm

Reviewed By: ashoat, atul

Subscribers: Adrian, karol-bisztyga, ashoat

Differential Revision: https://phab.comm.dev/D4453