Reduce the startup time for a Scala Unit-test by skipping Verilator compilation *and* design elaboration

[oharboe]

To interface to an existing test-system, I would like to launch a test from the command line hundreds of times by passing different test-input vector files on the command line.

E.g. given the input data in the "test.txt" file, I'd like to execute a test given:

$ sbt "test:runMain testmain test.txt"

The code below skips Verilator compilation(which is slow), but still performs design elaboration:

    Driver.run(() => new system(),
        "Vsystem")(c =>
          new system_Tests(c, args(0)))

[ekiwi]

You might be interested in this PR: ucb-bar/chiseltest#267