Cleanup harness generation and add missing vl_finish().

[ucbjrl]

I believe this solves the mystery of why some versions of verilator appear to fail with a:

Undefined symbols for architecture x86_64:
  "vl_finish(char const*, int, char const*)", referenced from:
      VL_FINISH_MT(char const*, int, char const*) in verilated.o
ld: symbol(s) not found for architecture x86_64

The harness generated by chisel3/src/main/resources/chisel3/top.cpp contains a definition of vl_finish() while the harness generated by chisel-testers/src/main/scala/chisel3/iotesters/VerilatorBackend.scala doesn't.

Tests run with pure Chisel3 and FIRRTL will pass without problems, while tests using chisel-testers will fail at the link stage.

This PR converts multiple codeBuffer.append()s with individual C++ source lines into two codeBuffer.append()s with triple quotes and adds the missing vl_finish() definition.

[chick]

This makes the harness code much easier to read.