public class Second {
  private WebDriver driver;
  private boolean acceptNextAlert = true;
  private StringBuffer verificationErrors = new StringBuffer();

  @BeforeClass
  public void beforeClass() {
  driver = new FirefoxDriver();
  driver.manage().timeouts().implicitlyWait(30, TimeUnit.SECONDS);
  driver.manage().window().maximize();
  }


 @Test
  public void testSecond() throws Exception {
  driver.get("url");
  System.out.println("test two");
  Thread.sleep(5000);

 }

 @AfterClass
 public void afterClass() throws Exception{
  driver.quit();
    String verificationErrorString = verificationErrors.toString();
    if (!"".equals(verificationErrorString)) {
      fail(verificationErrorString);
    }
 }
}

Este es un caso de prueba testNG que arroja Runtime Exception en driver.quit (). la prueba se pasa con éxito pero el navegador no se cierra una vez que se completa la prueba
Seguimiento de pila:

 FAILED CONFIGURATION: @AfterTest afterClass
java.lang.RuntimeException: Process refused to die after 10 seconds, and couldn't taskkill it: Unable to find executable for: taskkill
at org.openqa.selenium.os.ProcessUtils.killWinProcess(ProcessUtils.java:142)
at org.openqa.selenium.os.ProcessUtils.killProcess(ProcessUtils.java:81)
at org.openqa.selenium.os.UnixProcess$SeleniumWatchDog.destroyHarder(UnixProcess.java:248)
at org.openqa.selenium.os.UnixProcess$SeleniumWatchDog.access$2(UnixProcess.java:245)
at org.openqa.selenium.os.UnixProcess.destroy(UnixProcess.java:124)
at org.openqa.selenium.os.CommandLine.destroy(CommandLine.java:153)
at org.openqa.selenium.firefox.FirefoxBinary.quit(FirefoxBinary.java:259)
at org.openqa.selenium.firefox.internal.NewProfileExtensionConnection.quit(NewProfileExtensionConnection.java:202)
at org.openqa.selenium.firefox.FirefoxDriver$LazyCommandExecutor.quit(FirefoxDriver.java:376)
at org.openqa.selenium.firefox.FirefoxDriver.stopClient(FirefoxDriver.java:322)
at org.openqa.selenium.remote.RemoteWebDriver.quit(RemoteWebDriver.java:477)
at testNGTestCase.Second.afterClass(Second.java:54)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:601)
at org.testng.internal.MethodInvocationHelper.invokeMethod(MethodInvocationHelper.java:84)
at org.testng.internal.Invoker.invokeConfigurationMethod(Invoker.java:564)
at org.testng.internal.Invoker.invokeConfigurations(Invoker.java:213)
at org.testng.internal.Invoker.invokeConfigurations(Invoker.java:138)
at org.testng.TestRunner.afterRun(TestRunner.java:1014)
at org.testng.TestRunner.run(TestRunner.java:621)
at org.testng.SuiteRunner.runTest(SuiteRunner.java:334)
at org.testng.SuiteRunner.runSequentially(SuiteRunner.java:329)
at org.testng.SuiteRunner.privateRun(SuiteRunner.java:291)
at org.testng.SuiteRunner.run(SuiteRunner.java:240)
at org.testng.SuiteRunnerWorker.runSuite(SuiteRunnerWorker.java:52)
at org.testng.SuiteRunnerWorker.run(SuiteRunnerWorker.java:86)
at org.testng.TestNG.runSuitesSequentially(TestNG.java:1224)
at org.testng.TestNG.runSuitesLocally(TestNG.java:1149)
at org.testng.TestNG.run(TestNG.java:1057)
at org.testng.remote.RemoteTestNG.run(RemoteTestNG.java:111)
at org.testng.remote.RemoteTestNG.initAndRun(RemoteTestNG.java:204)
at org.testng.remote.RemoteTestNG.main(RemoteTestNG.java:175)
Caused by: java.lang.NullPointerException: Unable to find executable for: taskkill
at com.google.common.base.Preconditions.checkNotNull(Preconditions.java:250)
at org.openqa.selenium.os.UnixProcess.<init>(UnixProcess.java:62)
at org.openqa.selenium.os.CommandLine.<init>(CommandLine.java:38)
at org.openqa.selenium.os.WindowsUtils.killPID(WindowsUtils.java:172)
at org.openqa.selenium.os.ProcessUtils.killWinProcess(ProcessUtils.java:138)

He dado TcpTimedWaitDelay como 30 segundos.