core: fix import errors on clique crashes + empty blocks (#19544)

* core: fix import errors on clique crashes + empty blocks

* cosensus/clique, core: add test for the mirrored state issue

* core: address todo question wrt log count

* core: raise a louder warning for non-clique known blocks

core/chain_makers.go

@@ -71,6 +71,13 @@ func (b *BlockGen) SetNonce(nonce types.BlockNonce) {
 	b.header.Nonce = nonce
 }
 
+// SetDifficulty sets the difficulty field of the generated block. This method is
+// useful for Clique tests where the difficulty does not depend on time. For the
+// ethash tests, please use OffsetTime, which implicitly recalculates the diff.
+func (b *BlockGen) SetDifficulty(diff *big.Int) {
+	b.header.Difficulty = diff
+}
+
 // AddTx adds a transaction to the generated block. If no coinbase has
 // been set, the block's coinbase is set to the zero address.
 //