Reproduce #3798: TestModule.testForkGrouping doesn't work with TestNG

build.mill

@@ -771,6 +771,7 @@ object dist0 extends MillPublishJavaModule {
     build.contrib.jmh.testDep(),
     build.contrib.playlib.testDep(),
     build.contrib.playlib.worker("2.8").testDep(),
+    build.contrib.testng.testDep(),
     build.bsp.worker.testDep(),
     build.testkit.testDep()
   )

example/javalib/testing/4-test-grouping/build.mill

@@ -12,9 +12,53 @@ object foo extends JavaModule {
     )
     def testForkGrouping = discoveredTestClasses().grouped(1).toSeq
   }
+  object testng extends JavaTests with TestModule.TestNg {
+    def ivyDeps = super.ivyDeps() ++ Agg(
+      ivy"org.testng:testng:7.10.2"
+    )
+    def testForkGrouping = discoveredTestClasses().grouped(1).toSeq
+  }
 }
 
 /** See Also: foo/test/src/foo/HelloTests.java */
 /** See Also: foo/test/src/foo/WorldTests.java */
 
+/** Usage
+
+> mill foo.test
+
+> find out/foo/test/test.dest
+...
+out/foo/test/test.dest/foo.HelloTests.log
+out/foo/test/test.dest/foo.HelloTests/sandbox
+out/foo/test/test.dest/foo.WorldTests.log
+out/foo/test/test.dest/foo.WorldTests/sandbox
+out/foo/test/test.dest/test-report.xml
+
+> mill show foo.testng.discoveredTestClasses
+[
+  "foo.HelloTests",
+  "foo.WorldTests"
+]
+
+> mill show foo.testng.testForkGrouping
+[
+  [
+    "foo.HelloTests"
+  ],
+  [
+    "foo.WorldTests"
+  ]
+]
+
+> mill show foo.testng.test
+...
+..."fullyQualifiedName": "foo.HelloTests",
+...
+..."fullyQualifiedName": "foo.WorldTests",
+...
+
+*/
+
 //// SNIPPET:END
+

example/javalib/testing/4-test-grouping/foo/testng/src/foo/HelloTests.java

@@ -0,0 +1,16 @@
+package foo;
+
+import static org.testng.Assert.assertTrue;
+import org.testng.annotations.Test;
+
+public class HelloTests {
+
+  @Test
+  public void hello() throws Exception {
+    System.out.println("Testing Hello");
+    String result = new Foo().hello();
+    assertTrue(result.startsWith("Hello"));
+    Thread.sleep(1000);
+    System.out.println("Testing Hello Completed");
+  }
+}

example/javalib/testing/4-test-grouping/foo/testng/src/foo/WorldTests.java

@@ -0,0 +1,15 @@
+package foo;
+
+import static org.testng.Assert.assertTrue;
+import org.testng.annotations.Test;
+
+public class WorldTests {
+  @Test
+  public void world() throws Exception {
+    System.out.println("Testing World");
+    String result = new Foo().hello();
+    assertTrue(result.endsWith("World"));
+    Thread.sleep(1000);
+    System.out.println("Testing World Completed");
+  }
+}