[services] Backup - Fix unchanged variable names in unit tests scripts

Summary: Fixing unchanged variable names in integration tests script and unit tests scripts from DEV_MODE to SANDBOX.

Test Plan: cd services && yarn run-unit-tests backup

Reviewers: karol, tomek, max, ashoat, varun

Reviewed By: karol, tomek, varun

Subscribers: max, ashoat, tomek, adrian, atul, karol, abosh

Differential Revision: https://phab.comm.dev/D4502