Fix bug in partest.DPConsoleRunner

The bug was that we declared case classes like:
  case class CompFailed() extends NegTestState
but we used their companion objects like in:
  case _ => CompFailed

Interestingly, this bug was caught by compiling this code with dotty,
instead of `failureStates` getting inferred to be of type `AnyRef`, it
ended up being a union of object types, this allows dotty to realize our
subsequent pattern match on `failureStates` cannot possibly succeed:

-- Error: /home/smarter/opt/dotty/compiler/test/dotty/partest/DPConsoleRunner.scala
353 |          case CompFailedButWrongDiff() =>
    |          ^
    | missing parameter type for parameter x$1 of expanded function x$1 =>
    |   x$1 @unchecked match
    |     {
    |       case CompFailedButWrongDiff() =>
    |         nextTestActionFailing(s"output differs")
    |         true
    |       case _ =>
    |         false
    |     }, expected = ?
-- Error: /home/smarter/opt/dotty/compiler/test/dotty/partest/DPConsoleRunner.scala
353 |          case CompFailedButWrongDiff() =>
    |               ^^^^^^^^^^^^^^^^^^^^^^^^
    |Pattern type CompFailedButWrongDiff is neither a subtype nor a supertype of selector type CompSucceeded | CompFailedButWrongNErr | CompFailed | CompFailedButWrongDiff'where:    CompFailedButWrongDiff  is a class in method runNegTest
    |          CompFailedButWrongDiff' is a object in method runNegTest

Author: smarter

compiler/test/dotty/partest/DPConsoleRunner.scala

@@ -300,11 +300,11 @@ class DPTestRunner(testFile: File, suiteRunner: DPSuiteRunner) extends nest.Runn
     // Don't get confused, the neg test passes when compilation fails for at
     // least one round (optionally checking the number of compiler errors and
     // compiler console output)
-    case class CompFailed() extends NegTestState
+    case object CompFailed extends NegTestState
     // the neg test fails when all rounds return either of these:
     case class CompFailedButWrongNErr(expected: String, found: String) extends NegTestState
-    case class CompFailedButWrongDiff() extends NegTestState
-    case class CompSucceeded() extends NegTestState
+    case object CompFailedButWrongDiff extends NegTestState
+    case object CompSucceeded extends NegTestState
 
     def nerrIsOk(reason: String) = {
       val nerrFinder = """compilation failed with (\d+) errors""".r
@@ -350,7 +350,7 @@ class DPTestRunner(testFile: File, suiteRunner: DPSuiteRunner) extends nest.Runn
       if (existsNerr) false
       else {
         val existsDiff = failureStates.exists({
-          case CompFailedButWrongDiff() =>
+          case CompFailedButWrongDiff =>
             nextTestActionFailing(s"output differs")
             true
           case _ =>