fix: flaky TestReactor_Backfill test

[shotonoff]

Issue being fixed or feature implemented

Need to fix flaky TestReactor_Backfill test

What was done?

• refactored TestReactor_Backfill test
• appends peerID to peer-list only if a resulted light-block is not nil

How Has This Been Tested?

Unit-test

Breaking Changes

N/A

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated relevant unit/integration/functional/e2e tests
• I have made corresponding changes to the documentation

For repository code-owners and collaborators only

• I have assigned this pull request to a milestone