core: fix uncle creation in TestFastVsFullChains (#25476)

Co-authored-by: Felix Lange <fjl@twurst.com>

core/blockchain_test.go

@@ -759,9 +759,9 @@ func TestFastVsFullChains(t *testing.T) {
 				block.AddTx(tx)
 			}
 		}
-		// If the block number is a multiple of 5, add a few bonus uncles to the block
-		if i%5 == 5 {
-			block.AddUncle(&types.Header{ParentHash: block.PrevBlock(i - 1).Hash(), Number: big.NewInt(int64(i - 1))})
+		// If the block number is a multiple of 5, add an uncle to the block
+		if i%5 == 4 {
+			block.AddUncle(&types.Header{ParentHash: block.PrevBlock(i - 2).Hash(), Number: big.NewInt(int64(i))})
 		}
 	})
 	// Import the chain as an archive node for the comparison baseline