Share process pool for all @test

Author: liufengyun

compiler/test/dotty/tools/vulpix/RunnerOrchestration.scala

@@ -48,7 +48,9 @@ trait RunnerOrchestration {
   /** Running a `Test` class's main method from the specified `dir` */
   def runMain(classPath: String)(implicit summaryReport: SummaryReporting): Status =
     monitor.runMain(classPath)
+}
 
+object RunnerOrchestration {
   private val monitor = new RunnerMonitor
 
   /** The runner monitor object keeps track of child JVM processes by keeping
@@ -202,4 +204,4 @@ trait RunnerOrchestration {
     // On shutdown, we need to kill all runners:
     sys.addShutdownHook(killAll())
   }
-}
+}