Add test for backfill service with disabled backfill

[GarmashAlex]

What type of PR is this?

Bug fix

What does this PR do? Why is it needed?

This PR fixes linter errors in the beacon-chain/sync/backfill/service_test.go file. The linter was reporting two errors:

1. undefined: require.False
2. srv.isBackfilling undefined (type *Service has no field or method isBackfilling)
   The PR adds a test case to verify the behavior of the backfill service when backfill is explicitly disabled. The test ensures that the service properly respects the disabled configuration and does not attempt to perform backfill operations.

Which issues(s) does this PR fix?

Fixes linter errors in the backfill service test file.

Other notes for review

The test adds proper verification of the backfill service's state when backfill is disabled, which was missing from the test suite.

Acknowledgements

• I have read CONTRIBUTING.md.
• I have included a uniquely named changelog fragment file.
• I have added a description to this PR with sufficient context for reviewers to understand this PR.

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