Speed up tests with testForkGrouping (#4650)

Author: jackkoenig

build.mill

@@ -283,6 +283,8 @@ trait Chisel extends CrossSbtModule with HasScala2MacroAnno with HasScala2Plugin
     // Suppress Scala 3 behavior requiring explicit types on implicit definitions
     // Note this must come before the -Wconf is warningSuppression
     override def scalacOptions = Task { super.scalacOptions() :+ "-Wconf:cat=other-implicit-type:s" }
+
+    override def testForkGrouping = discoveredTestClasses().grouped(8).toSeq
   }
 }
 

integration-tests/package.mill

@@ -22,5 +22,7 @@ trait IntegrationTests extends CrossSbtModule with HasScala2Plugin with HasCommo
   object test extends SbtTests with TestModule.ScalaTest with ScalafmtModule {
     override def moduleDeps = super.moduleDeps :+ chisel().test
     def ivyDeps = Agg(v.scalatest, v.scalacheck)
+
+    override def testForkGrouping = discoveredTestClasses().grouped(8).toSeq
   }
 }