validation: Disable CheckForkWarningConditions for background chainstate

The comparison of m_best_invalid with the tip of the respective chainstate
makes no sense for the background chainstate, and can lead to incorrect
error messages.

src/validation.cpp

@@ -2020,7 +2020,8 @@ void Chainstate::CheckForkWarningConditions()
 
     // Before we get past initial download, we cannot reliably alert about forks
     // (we assume we don't get stuck on a fork before finishing our initial sync)
-    if (m_chainman.IsInitialBlockDownload()) {
+    // Also not applicable to the background chainstate
+    if (m_chainman.IsInitialBlockDownload() || this->GetRole() == ChainstateRole::BACKGROUND) {
         return;
     }