Write test harness in Chisel

This is an unavoidably invasive commit, because it affects the unit tests
(which formerly exited using stop()), the test harness Verilog generator
(since it is no longer necessary), and the DRAM model (since it is no
longer connected).  However, this should substantially reduce the effort
of building test harnesses in the future, since manual or semi-automatic
Verilog writing should no longer be necessary.  Furthermore, there is now
very little duplication of effort between the Verilator and VCS test
harnesses.

This commit removes support for DRAMsim, which is a bit of an unfortunate
consequence.  The main blocker is the lack of Verilog parameterization for
BlackBox.  It would be straightforward to revive DRAMsim once support for
that feature is added to Chisel and FIRRTL.  But that might not even be
necessary, as we move towards synthesizable DRAM models and FAME-1
transformations.

project/build.scala

@@ -16,7 +16,7 @@ object BuildSettings extends Build {
     libraryDependencies ++= Seq("org.scala-lang" % "scala-reflect" % scalaVersion.value)
   )
 
-  lazy val chisel = project in file("chisel" + sys.env.getOrElse("CHISEL_VERSION", 3))
+  lazy val chisel = project in file("chisel3")
   lazy val cde        = project in file("context-dependent-environments")
   lazy val hardfloat  = project.dependsOn(chisel)
   lazy val junctions  = project.dependsOn(chisel, cde)