Merge branch 'testers2' into testers2+interprocesstester

# Conflicts:
#	build.sbt
#	src/main/scala/chisel3/tester/FirrterpreterBackend.scala
#	src/main/scala/chisel3/tester/Testers2.scala
#	src/main/scala/chisel3/tester/ThreadedBackend.scala