First cut at refactoring unittests into a top-level utility. Individual tests co-located with their DUT. No functional changes.

src/main/scala/rocketchip/TestConfigs.scala

@@ -7,9 +7,8 @@ import uncore.tilelink._
 import uncore.coherence._
 import uncore.agents._
 import uncore.devices.NTiles
-import uncore.unittests._
+import unittest._
 import junctions._
-import junctions.unittests._
 import scala.collection.mutable.LinkedHashSet
 import scala.collection.immutable.HashMap
 import cde.{Parameters, Config, Dump, Knob, CDEMatchError}