[2.x] Replace commented System.err.println with proper logger.debug (#8693)

**Problem**
The writeSuite() method in JUnitXmlTestsListener had a commented-out System.err.println statement with a TODO to use proper logging.

build.sbt

@@ -424,7 +424,9 @@ lazy val testingProj = (project in file("testing"))
       launcherInterface,
       sjsonNewScalaJson.value,
       sjsonNewCore.value,
+      scalaVerify % Test,
     ),
+    testFrameworks += TestFramework("verify.runner.Framework"),
     conflictWarning := ConflictWarning.disable,
     contrabandSettings,
     mimaSettings,

testing/src/main/scala/sbt/JUnitXmlTestsListener.scala

@@ -278,8 +278,9 @@ class JUnitXmlTestsListener(val targetDir: File, legacyTestReport: Boolean, logg
     } else {
       new File(targetDir, s"TEST-${normalizeName(withTestSuite(_.name))}.xml").getAbsolutePath
     }
-    // TODO would be nice to have a logger and log this with level debug
-    // System.err.println("Writing JUnit XML test report: " + file)
+    if (logger != null) {
+      logger.debug(s"writing JUnit XML test report: $file")
+    }
     val testSuiteResult = withTestSuite(_.stop())
     XML.save(file, testSuiteResult, "UTF-8", xmlDecl = true, null)
     testSuite.remove()

testing/src/test/scala/sbt/JUnitXmlTestsListenerSpec.scala

@@ -0,0 +1,105 @@
+/*
+ * sbt
+ * Copyright 2023, Scala center
+ * Copyright 2011 - 2022, Lightbend, Inc.
+ * Copyright 2008 - 2010, Mark Harrah
+ * Licensed under Apache License 2.0 (see LICENSE)
+ */
+
+package sbt
+
+import java.io.File
+import java.util.concurrent.atomic.AtomicReference
+
+import testing.{ Event as TEvent, OptionalThrowable, Status as TStatus, TestSelector }
+import util.{ AbstractLogger, Level, ControlEvent, LogEvent }
+import sbt.protocol.testing.TestResult
+import verify.BasicTestSuite
+
+object JUnitXmlTestsListenerSpec extends BasicTestSuite:
+
+  test("JUnitXmlTestsListener should log debug message when writing test report"):
+    val tempDir = File.createTempFile("junit-test", "")
+    tempDir.delete()
+    tempDir.mkdirs()
+    try
+      val loggedMessages = new AtomicReference[List[String]](Nil)
+      val mockLogger = new AbstractLogger:
+        def getLevel: Level.Value = Level.Debug
+        def setLevel(newLevel: Level.Value): Unit = ()
+        def getTrace: Int = 0
+        def setTrace(flag: Int): Unit = ()
+        def successEnabled: Boolean = false
+        def setSuccessEnabled(flag: Boolean): Unit = ()
+        def control(event: ControlEvent.Value, message: => String): Unit = ()
+        def logAll(events: Seq[LogEvent]): Unit = ()
+        def trace(t: => Throwable): Unit = ()
+        def success(message: => String): Unit = ()
+        def log(level: Level.Value, message: => String): Unit =
+          if level == Level.Debug then loggedMessages.updateAndGet(_ :+ message)
+
+      val listener = new JUnitXmlTestsListener(tempDir, false, mockLogger)
+      listener.doInit()
+      listener.startGroup("TestSuite")
+
+      // Create a test event
+      val testEvent = new TEvent:
+        def fullyQualifiedName = "TestSuite.testMethod"
+        def duration() = 100L
+        def status = TStatus.Success
+        def fingerprint = null
+        def selector = new TestSelector("testMethod")
+        def throwable = new OptionalThrowable()
+
+      listener.testEvent(sbt.TestEvent(Seq(testEvent)))
+
+      // End the group to trigger writeSuite()
+      listener.endGroup("TestSuite", TestResult.Passed)
+
+      // Verify that the debug message was logged
+      val messages = loggedMessages.get()
+      assert(
+        messages.exists(_.contains("writing JUnit XML test report")),
+        s"Expected log message containing 'writing JUnit XML test report', but got: $messages"
+      )
+      assert(
+        messages.exists(_.contains("TEST-TestSuite.xml")),
+        s"Expected log message containing 'TEST-TestSuite.xml', but got: $messages"
+      )
+    finally
+      // Cleanup
+      if tempDir.exists() then
+        tempDir.listFiles().foreach(_.delete())
+        tempDir.delete()
+
+  test("JUnitXmlTestsListener should handle null logger gracefully"):
+    val tempDir = File.createTempFile("junit-test", "")
+    tempDir.delete()
+    tempDir.mkdirs()
+    try
+      val listener = new JUnitXmlTestsListener(tempDir, false, null)
+      listener.doInit()
+      listener.startGroup("TestSuite")
+
+      val testEvent = new TEvent:
+        def fullyQualifiedName = "TestSuite.testMethod"
+        def duration() = 100L
+        def status = TStatus.Success
+        def fingerprint = null
+        def selector = new TestSelector("testMethod")
+        def throwable = new OptionalThrowable()
+
+      listener.testEvent(sbt.TestEvent(Seq(testEvent)))
+
+      // Should not throw when logger is null
+      listener.endGroup("TestSuite", TestResult.Passed)
+
+      // Verify XML file was still created
+      val xmlFile = new File(tempDir, "TEST-TestSuite.xml")
+      assert(xmlFile.exists(), "XML file should be created even when logger is null")
+    finally
+      if tempDir.exists() then
+        tempDir.listFiles().foreach(_.delete())
+        tempDir.delete()
+
+end JUnitXmlTestsListenerSpec