也遇到了这个问题。mojo规则失败了，因为它示例化了一个AbstractMojoTestCase的匿名实现，然后它会查找哪个，然后尝试创建一个输入流，在这里加载数据：InputStream is = getClass().getResourceAsStream( "/" + getPluginDescriptorLocation() );。这（getPluginDescriptorLocation()）只是一个硬编码的文件字符串"META-INF/maven/plugin.xml"。
我通过添加以下依赖项来解决这个问题：

<dependency>
  <groupId>org.apache.maven</groupId>
  <artifactId>maven-compat</artifactId>
  <version>3.5.0</version>
</dependency>

我不知道为什么这解决了这个问题。该文件仍然不存在（我检查了原始源代码和target目录，但它都不在）。因此，需要额外的搜索来弄清楚为什么这样做。
下面是我的依赖项以供参考。

<dependency>
  <groupId>org.apache.maven</groupId>
  <artifactId>maven-plugin-api</artifactId>
  <version>3.5.0</version>
</dependency>
<dependency>
  <groupId>org.twdata.maven</groupId>
  <artifactId>mojo-executor</artifactId>
  <version>2.3.0</version>
</dependency>
<dependency>
  <groupId>org.apache.maven.plugin-tools</groupId>
  <artifactId>maven-plugin-annotations</artifactId>
  <version>3.5</version>
  <scope>provided</scope>
</dependency>
<dependency>
  <groupId>org.eclipse.aether</groupId>
  <artifactId>aether-api</artifactId>
  <version>1.1.0</version>
</dependency>
<dependency>
  <groupId>org.apache.maven</groupId>
  <artifactId>maven-core</artifactId>
  <version>3.5.0</version>
</dependency>
<dependency>
  <groupId>org.apache.maven</groupId>
  <artifactId>maven-compat</artifactId>
  <version>3.5.0</version>
</dependency>
<!--Test Dependencies-->
<dependency>
  <groupId>junit</groupId>
  <artifactId>junit</artifactId>
  <version>4.12</version>
  <scope>test</scope>
</dependency>
<dependency>
  <groupId>org.apache.maven.plugin-testing</groupId>
  <artifactId>maven-plugin-testing-harness</artifactId>
  <version>3.3.0</version>
  <scope>test</scope>
</dependency>

我也有同样的问题。通过添加正确的依赖关系解决了它。重要的是，标记为由harness maven插件提供的依赖项被添加到测试范围：

<properties>
  <dependency.maven.version>3.3.9</dependency.maven.version>
  <!-- and others -->
</properties>

<dependencies>
  <!-- your dependencies -->

  <!-- maven plugin dependencies -->
  <dependency>
    <groupId>org.apache.maven</groupId>
    <artifactId>maven-plugin-api</artifactId>
    <version>${dependency.maven.version}</version>
  </dependency>
  <dependency>
    <groupId>org.apache.maven</groupId>
    <artifactId>maven-core</artifactId>
    <version>${dependency.maven.version}</version>
  </dependency>
  <dependency>
    <groupId>org.apache.maven</groupId>
    <artifactId>maven-model</artifactId>
    <version>${dependency.maven.version}</version>
  </dependency>
  <dependency>
    <groupId>org.apache.maven</groupId>
    <artifactId>maven-artifact</artifactId>
    <version>${dependency.maven.version}</version>
  </dependency>

  <!-- test dependencies -->
  <dependency>
    <groupId>junit</groupId>
    <artifactId>junit</artifactId>
    <version>${dependency.junit.version}</version>
    <scope>test</scope>
  </dependency>
  <dependency>
    <groupId>org.apache.maven.plugin-testing</groupId>
    <artifactId>maven-plugin-testing-harness</artifactId>
    <version>3.3.0</version>
    <scope>test</scope>
  </dependency>
  <dependency>
    <groupId>org.apache.maven</groupId>
    <artifactId>maven-aether-provider</artifactId>
    <version>${dependency.maven.version}</version>
    <scope>test</scope>
  </dependency>
  <dependency>
    <groupId>org.apache.maven</groupId>
    <artifactId>maven-compat</artifactId>
    <version>${dependency.maven.version}</version>
    <scope>test</scope>
  </dependency>

  <!-- provided maven dependencies -->
  <dependency>
    <groupId>org.apache.maven.plugin-tools</groupId>
    <artifactId>maven-plugin-annotations</artifactId>
    <version>3.3</version>
    <scope>provided</scope>
  </dependency>

</dependencies>

您的Test类应该如下所示

public class YourMojoTest {

  @Rule
  public MojoRule rule = new MojoRule();

  @Test
  public void shouldExecuteMojo() throws Exception {
    // given
    File testPom = new File(PlexusTestCase.getBasedir(), "src/test/resources/pom.xml");

    final Properties properties = new Properties();
    final MavenProject mavenProject = Mockito.mock(MavenProject.class);
    Mockito.when(mavenProject.getProperties()).thenReturn(properties);

    // when
    YourMojo mojo = (YourMojo) rule.lookupMojo("fetch", testPom);
    assertNotNull(mojo);
    mojo.setProject(mavenProject);
    mojo.execute();

    // then
    // do your tests ...
  }
}

希望这有助于其他挣扎在同样的问题

通过添加maven兼容性库修复了此问题：-

<dependency>
            <groupId>org.apache.maven</groupId>
            <artifactId>maven-compat</artifactId>
            <version>${maven.version}</version>
            <scope>test</scope>
        </dependency>

POM文件的加载看起来不太好：
下面的代码来自示例：

File pom = rule.getTestFile( "src/test/resources/unit/project-to-test/pom.xml" );

这就是你所拥有的

File pom = new File(POM_FILE_NAME);

除此之外，您还有一个不同的位置，看起来像这样：

private static final String POM_FILE_NAME = "/path/to/pom.xml";

但是pom文件的位置应该是：

private static final String POM_FILE_NAME = "src/test/resources/pom.xml";