maven 的 pom.xml 是这样的

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>
  <groupId>demo</groupId>
  <artifactId>demo</artifactId>
  <version>0.0.1-SNAPSHOT</version>

  <properties>
    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
  </properties>

  <dependencies>
    <dependency>
        <groupId>org.seleniumhq.selenium</groupId>
        <artifactId>selenium-java</artifactId>
        <version>3.9.1</version>
    </dependency> 
    <dependency>
        <groupId>org.testng</groupId>
        <artifactId>testng</artifactId>
        <version>6.14.2</version>
        <scope>test</scope>
    </dependency>
  </dependencies>

  <build>
    <pluginManagement>
        <plugins>
            <plugin>
                 <groupId>org.apache.maven.plugins</groupId>
                 <artifactId>maven-surefire-plugin</artifactId>
                 <version>2.20.1</version>
                 <configuration>
                     <suitexmlfiles>
                         <suitexmlfile>testng.xml</suitexmlfile>
                     </suitexmlfiles>
                </configuration> 
            </plugin>

            <plugin>  
                <groupId>org.apache.maven.plugins</groupId>  
                <artifactId>maven-compiler-plugin</artifactId>  
                <configuration>  
                    <encoding>UTF-8</encoding>  
                </configuration>  
            </plugin>
        </plugins>
    </pluginManagement>
  </build>
</project>

testng.xml 是这样，放在根目录下

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE suite SYSTEM "http://testng.org/testng-1.0.dtd">
<suite name="Suite">
  <test thread-count="5" name="Test">
    <classes>
      <class name="demo.Sample"/>
    </classes>
  </test> <!-- Test -->
</suite> <!-- Suite -->

然而我运行 mvn test 的时候什么都没有运行，好像是 maven 没有找到 testng.xml, 实在不清楚怎么回事，求解答