Allow internal tests to set whether or not a VCD should be dumped

[edwardcwang]

Type of issue: other enhancement

Impact: no impact to user code

Development Phase: request

Other information: Some long running tests (as noticed in #1088, though exhaustive testing may not be the right approach in a unit test) may want to be able to control or suppress if a VCD is generated.

Chisel's TestDriver calls FIRRTL's verilogToCpp which has VCD generation turned on by default.

https://github.com/freechipsproject/chisel3/blob/7da0d5f8704b25a94e25cd9638c77695ae081d7f/src/main/scala/chisel3/testers/TesterDriver.scala#L60

https://github.com/freechipsproject/firrtl/blob/a7cf6ff3416a11088d811a435ba71fd36b191fb4/src/main/scala/firrtl/util/BackendCompilationUtilities.scala#L101

[seldridge]

My specific annoyance is that assertTesterPasses is generating VCDs by default. Repeated runs of sbt test keep filling up test_run_dir with a lot of VCD files that aren't being checked.

[jackkoenig]

Turn off VCD generation by default, add flag to turn them on? It's also probably reasonable for developers to just change the Scala to turn it back on.