That did the job

         <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-checkstyle-plugin</artifactId>
                <version>3.6.0</version>
                <executions>
                    <execution>
                        <phase>validate</phase>
                        <goals>
                            <goal>check</goal>
                        </goals>
                        <configuration>
                            <configLocation>checkstyle.xml</configLocation>
                            <sourceDirectories>
                                <sourceDirectory>${project.basedir}/src/test/java</sourceDirectory>
                            </sourceDirectories>
                            <failOnViolation>true</failOnViolation>
                        </configuration>
                    </execution>
                </executions>
            </plugin>

Thanks to @Turing85