Print done message from test framework to stdout

We are returning the doneMessage from the test framework
as value of the test task, but we are not printing it.
Moreover, the output is shortcircuited when a task fails,
so it's not passed to the `test` command.
This PR uses `ctx.log.outputStream.println(doneMessage)` to
print the message before returning it so users can clearly see it
in case of both success and failure.
This was discovered because of the way weaver works on Scala.js
Since the `TestRunnerTests` are on `scalalib` and can't use Scala.js
code, I implemented two dummy `sbt.testing.Framework`s which just
return a fixed `done` message.

build.sc

@@ -397,6 +397,7 @@ trait MillBaseTestsModule extends MillJavaModule with TestModule {
       s"-DTEST_SCALANATIVE_VERSION=${Deps.Scalanative_0_4.scalanativeVersion}",
       s"-DTEST_UTEST_VERSION=${Deps.utest.dep.version}",
       s"-DTEST_SCALATEST_VERSION=${Deps.TestDeps.scalaTest.dep.version}",
+      s"-DTEST_TEST_INTERFACE_VERSION=${Deps.sbtTestInterface.dep.version}",
       s"-DTEST_ZIOTEST_VERSION=${Deps.TestDeps.zioTest.dep.version}",
       s"-DTEST_ZINC_VERSION=${Deps.zinc.dep.version}"
     )

scalalib/test/resources/testrunner/doneMessageFailure/src/DoneMessageFailureFramework.scala

@@ -0,0 +1,42 @@
+package mill.scalalib
+
+import sbt.testing._
+
+class DoneMessageFailureFramework extends Framework {
+  def fingerprints() = Array.empty
+  def name() = "DoneMessageFailureFramework"
+  def runner(
+      args: Array[String],
+      remoteArgs: Array[String],
+      testClassLoader: ClassLoader
+  ): Runner = new Runner {
+    def args() = Array.empty
+    def done() = "test failure done message"
+    def remoteArgs() = Array.empty
+    def tasks(taskDefs: Array[TaskDef]) = Array(new Task {
+      def taskDef(): TaskDef = null
+      def execute(
+          eventHandler: EventHandler,
+          loggers: Array[Logger]
+      ): Array[Task] = {
+        eventHandler.handle(new Event {
+
+          override def fullyQualifiedName(): String = "foo.bar"
+
+          override def fingerprint(): Fingerprint = new Fingerprint {}
+
+          override def selector(): Selector = new TestSelector("foo.bar")
+
+          override def status(): Status = Status.Failure
+
+          override def throwable(): OptionalThrowable = new OptionalThrowable()
+
+          override def duration(): Long = 0L
+
+        })
+        Array.empty
+      }
+      def tags = Array.empty
+    })
+  }
+}

scalalib/test/resources/testrunner/doneMessageSuccess/src/DoneMessageSuccessFramework.scala

@@ -0,0 +1,18 @@
+package mill.scalalib
+
+import sbt.testing._
+
+class DoneMessageSuccessFramework extends Framework {
+  def fingerprints() = Array.empty
+  def name() = "DoneMessageSuccessFramework"
+  def runner(
+      args: Array[String],
+      remoteArgs: Array[String],
+      testClassLoader: ClassLoader
+  ): Runner = new Runner {
+    def args() = Array.empty
+    def done() = "test success done message"
+    def remoteArgs() = Array.empty
+    def tasks(taskDefs: Array[TaskDef]) = Array.empty
+  }
+}

scalalib/test/src/mill/scalalib/TestRunnerTests.scala

@@ -2,10 +2,13 @@ package mill.scalalib
 
 import mill.{Agg, T}
 
+import mill.api.Result
 import mill.util.{TestEvaluator, TestUtil}
 import utest._
 import utest.framework.TestPath
 
+import java.io.{ByteArrayOutputStream, PrintStream}
+
 object TestRunnerTests extends TestSuite {
   object testrunner extends TestUtil.BaseModule with ScalaModule {
     override def millSourcePath = TestUtil.getSrcPathBase() / millOuterCtx.enclosing.split('.')
@@ -28,6 +31,20 @@ object TestRunnerTests extends TestSuite {
       }
     }
 
+    trait DoneMessage extends ScalaTests {
+      override def ivyDeps = T {
+        super.ivyDeps() ++ Agg(
+          ivy"org.scala-sbt:test-interface:${sys.props.getOrElse("TEST_TEST_INTERFACE_VERSION", ???)}"
+        )
+      }
+    }
+    object doneMessageSuccess extends DoneMessage {
+      def testFramework = "mill.scalalib.DoneMessageSuccessFramework"
+    }
+    object doneMessageFailure extends DoneMessage {
+      def testFramework = "mill.scalalib.DoneMessageFailureFramework"
+    }
+
     object ziotest extends ScalaTests with TestModule.ZioTest {
       override def ivyDeps = T {
         super.ivyDeps() ++ Agg(
@@ -42,11 +59,13 @@ object TestRunnerTests extends TestSuite {
 
   def workspaceTest[T](
       m: TestUtil.BaseModule,
+      errStream: PrintStream = System.err,
+      outStream: PrintStream = System.out,
       resourcePath: os.Path = resourcePath
   )(t: TestEvaluator => T)(
       implicit tp: TestPath
   ): T = {
-    val eval = new TestEvaluator(m)
+    val eval = new TestEvaluator(m, errStream = errStream, outStream = outStream)
     os.remove.all(m.millSourcePath)
     os.remove.all(eval.outPath)
     os.makeDir.all(m.millSourcePath / os.up)
@@ -88,6 +107,24 @@ object TestRunnerTests extends TestSuite {
         }
       }
 
+      "doneMessage" - {
+        test("failure") {
+          val outStream = new ByteArrayOutputStream()
+          workspaceTest(testrunner, outStream = new PrintStream(outStream, true)) { eval =>
+            val Left(Result.Failure(msg, _)) = eval(testrunner.doneMessageFailure.test())
+            val stdout = new String(outStream.toByteArray)
+            assert(stdout.contains("test failure done message"))
+          }
+        }
+        test("success") {
+          val outStream = new ByteArrayOutputStream()
+          workspaceTest(testrunner, outStream = new PrintStream(outStream, true)) { eval =>
+            val Right(_) = eval(testrunner.doneMessageSuccess.test())
+            val stdout = new String(outStream.toByteArray)
+            assert(stdout.contains("test success done message"))
+          }
+        }
+      }
       "ScalaTest" - {
         test("scalatest.test") {
           workspaceTest(testrunner) { eval =>

testrunner/src/mill/testrunner/TestRunnerUtils.scala

@@ -156,6 +156,8 @@ import scala.jdk.CollectionConverters.IteratorHasAsScala
       runner.done()
     }
 
+    ctx.log.outputStream.println(doneMessage)
+
     val results = for (e <- events.iterator().asScala) yield {
       val ex =
         if (e.throwable().isDefined) Some(e.throwable().get) else None