[mill] Get unit testing working

Fixes https://github.com/chipsalliance/chisel/issues/4353.

Another step toward replacing SBT.

I went ahead and used the way of defining tests in Mill rather than doing it more manually. This ensures we get the SBT default src/test/scala and src/test/resource directories for free. It's also just more idiomatic as this is what the Mill examples do.

These tests can be run in parallel with ./mill -j0 __.test. I also figured out how to create "aggregate commands" that run in parallel (if you tell mill to use parallelism with -j):

object utils {
  // Copied from https://github.com/com-lihaoyi/mill/blob/4caadfb62581b4c1772c14c6f8dc8963011eaec7/main/define/src/mill/define/Task.scala#L55
  // This makes it possible to run multiple tasks in parallel (when passing `-j <n>` to mill)
  // Mill's source has MIT license
  class Sequence[+T](inputs0: Seq[Task[T]]) extends Task[Seq[T]] {
    val inputs: Seq[Task[_]] = inputs0
    def evaluate(ctx: mill.api.Ctx): mill.api.Result[Seq[T]] = {
      for (i <- 0 until ctx.args.length)
        yield ctx.args(i).asInstanceOf[T]
    }
  }
  // end copied from Mill

  // Sugar API over Sequence
  def parallelize[T](tasks: Seq[Task[T]]): Task[Seq[T]] = new Sequence(tasks)
}

object test extends TaskModule {

  override def defaultCommandName() = "run"

  val tasks = Seq(
    svsim(v.scalaVersion).test.test(),
    firrtl(v.scalaVersion).test.test(),
    chisel(v.scalaVersion).test.test(),
    integrationTests(v.scalaVersion).test.test()
  ) 
  def run() = T.command {
    utils.parallelize(tasks)()
  }
}

I don't think we need this yet, but if we ever want command "aliases", this approach might be useful.

Contributor Checklist

[ ] Did you add Scaladoc to every public function/method?
[ ] Did you add at least one test demonstrating the PR?
[x] Did you delete any extraneous printlns/debugging code?
[x] Did you specify the type of improvement?
[ ] Did you add appropriate documentation in docs/src?
[x] Did you request a desired merge strategy?
[ ] Did you add text to be included in the Release Notes for this change?

Type of Improvement

Internal or build-related (includes code refactoring/cleanup)

Desired Merge Strategy

Squash

Release Notes

Reviewer Checklist (only modified by reviewer)

[ ] Did you add the appropriate labels? (Select the most appropriate one based on the "Type of Improvement")
[ ] Did you mark the proper milestone (Bug fix: 3.6.x, 5.x, or 6.x depending on impact, API modification or big change: 7.0)?
[ ] Did you review?
[ ] Did you check whether all relevant Contributor checkboxes have been checked?
[ ] Did you do one of the following when ready to merge:
- [ ] Squash: You/ the contributor Enable auto-merge (squash), clean up the commit message, and label with Please Merge.
- [ ] Merge: Ensure that contributor has cleaned up their commit history, then merge with Create a merge commit.

chipsalliance / chisel