test(statesync): fix flaky TestReactor_Backfill

[lklimek]

Issue being fixed or feature implemented

TestReactor_Backfill is flaky.

Due to some timing constraints, sometimes backfill goes as low as height 8, while it should end on height 10.
This is caused by failure testing logic effectively "delaying" some blocks at higher heights.

What was done?

1. Added mechanism to reject blocks below 10 to the tests.
2. Small refactoring of backfill error handling

How Has This Been Tested?

#!/bin/bash -ex

while true; do
    go test -count 1 -timeout 60s ./internal/statesync/ -test.v -run TestReactor_Backfill 2>&1 | tee log.log
    if grep -q FAIL log.log; then
        echo "Test failed, stopping."
        break
    fi
    echo "================ Test passed, running again ================"
done

Breaking Changes

None

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

[shumkov]

👍