feat: support test case selection in TestNG framework (#4506)

needed for: scalameta/metals#7200

This allows to select and run single tests in Metals using
TestNGFramework.

![TestNGSingleTest](https://github.com/user-attachments/assets/9038111e-ef84-46cd-9168-9a34a6112d5a)

Previously only whole suite selection was possible.

Tested it manually as visible on the GIF above.

---------

Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>

contrib/testng/src/mill/testng/ResultEvent.java

@@ -27,7 +27,7 @@ public Fingerprint fingerprint() {
       }
 
       public Selector selector() {
-        return new SuiteSelector();
+        return new TestSelector(testNGResult.getName());
       }
 
       public Status status() {

contrib/testng/src/mill/testng/TestNGRunner.java

@@ -1,12 +1,17 @@
 package mill.testng;
 
 import com.beust.jcommander.JCommander;
+import java.util.ArrayList;
+import java.util.List;
 import org.testng.CommandLineArgs;
 import sbt.testing.EventHandler;
 import sbt.testing.Logger;
 import sbt.testing.Runner;
+import sbt.testing.Selector;
+import sbt.testing.SuiteSelector;
 import sbt.testing.Task;
 import sbt.testing.TaskDef;
+import sbt.testing.TestSelector;
 
 class TestNGTask implements Task {
 
@@ -55,11 +60,25 @@ public Task[] tasks(TaskDef[] taskDefs) {
       CommandLineArgs cliArgs = new CommandLineArgs();
       new JCommander(cliArgs).parse(args); // args is an output parameter of the constructor!
       cliArgs.testClass = taskDefs[i].fullyQualifiedName();
+      cliArgs.commandLineMethods =
+          testMethods(taskDefs[i].selectors(), taskDefs[i].fullyQualifiedName());
       returnTasks[i] = new TestNGTask(taskDefs[i], testClassLoader, cliArgs);
     }
     return returnTasks;
   }
 
+  private List<String> testMethods(Selector[] selectors, String testClass) {
+    ArrayList<String> testMethods = new ArrayList<String>();
+    for (int i = 0; i < selectors.length; i += 1) {
+      if (selectors[i] instanceof TestSelector) {
+        testMethods.add(testClass + "." + ((TestSelector) selectors[i]).testName());
+      } else if (selectors[i] instanceof SuiteSelector) {
+        return new ArrayList<String>();
+      }
+    }
+    return testMethods;
+  }
+
   public String done() {
     return null;
   }