Bump difftest: auto topIO connection and refactor checker

Author: klin02

difftest

@@ -45,8 +45,7 @@ class NutShellSim extends Module with HasDiffTestInterfaces {
 
   val uart = IO(new UARTIO)
   uart <> mmio.io.uart
-  override def connectTopIOs(difftest: DifftestTopIO): Seq[Data] = {
+  override def connectTopIOs(difftest: DifftestTopIO): Unit = {
     difftest.uart <> uart
-    Seq.empty
   }
 }

src/main/scala/sim/NutShellSim.scala

@@ -39,9 +39,6 @@ class Top extends Module {
 class FpgaDiffTop extends NutShell()(NutCoreConfig(FPGADifftest = true)) with HasDiffTestInterfaces {
   override def desiredName: String = "NutShell"
   override def cpuName: Option[String] = Some("NutShell")
-  override def connectTopIOs(difftest: DifftestTopIO): Seq[Data] = {
-    Seq(io)
-  }
 }
 
 object TopMain extends App {