community build: add support for testOnlyDependencies

This allows specifying dependencies that are needed only to build the
test suite of a community build project. The evaluation of this list
is delayed to permit what would otherwise be circular dependencies.

The addition of this capability is motivated by recent versions of
ScalaTest, which since version 3.2.4 has test-only dependencies on
scalatestplus-junit and scalatestplus-testng, which in turn both
depend on ScalaTest itself.

Author: griggt

community-build/src/scala/dotty/communitybuild/projects.scala

@@ -40,6 +40,7 @@ sealed trait CommunityProject:
   val publishCommand: String
   val docCommand: String
   val dependencies: List[CommunityProject]
+  val testOnlyDependencies: () => List[CommunityProject]
   val binaryName: String
   val runCommandsArgs: List[String] = Nil
   val requiresExperimental: Boolean
@@ -87,6 +88,7 @@ final case class MillCommunityProject(
     project: String,
     baseCommand: String,
     dependencies: List[CommunityProject] = Nil,
+    testOnlyDependencies: () => List[CommunityProject] = () => Nil,
     ignoreDocs: Boolean = false,
     requiresExperimental: Boolean = false,
     ) extends CommunityProject:
@@ -104,6 +106,7 @@ final case class SbtCommunityProject(
     sbtTestCommand: String,
     extraSbtArgs: List[String] = Nil,
     dependencies: List[CommunityProject] = Nil,
+    testOnlyDependencies: () => List[CommunityProject] = () => Nil,
     sbtPublishCommand: String = null,
     sbtDocCommand: String = null,
     scalacOptions: List[String] = SbtCommunityProject.scalacOptions,

community-build/test/scala/dotty/communitybuild/CommunityBuildTest.scala

@@ -27,6 +27,7 @@ abstract class CommunityBuildTest:
       )
       return
     self.dependencies.foreach(_.publish())
+    self.testOnlyDependencies().foreach(_.publish())
     suite.test(self)
 
   /** Build the given project with the published local compiler and sbt plugin.