The easy way

You can produce a jar which will include your test classes and resources.

<project>
  ...
  <build>
    <plugins>
      ...
      <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-jar-plugin</artifactId>
        <version>3.2.1</version>
        <executions>
          <execution>
            <goals>
              <goal>test-jar</goal>
            </goals>
          </execution>
        </executions>
      </plugin>
      ...
    </plugins>
  </build>
  ...
</project>

To reuse this artifact in an other project, you must declare this dependency with type test-jar :

<project>
  ...
  <dependencies>
    <dependency>
      <groupId>groupId</groupId>
      <artifactId>artifactId</artifactId>
      <classifier>tests</classifier>
      <type>test-jar</type>
      <version>version</version>
      <scope>test</scope>
    </dependency>
  </dependencies>
  ...
</project>

Based on such configuration there will be two jar files generated. The first one contains the classes from src/main/java whereas the second one will contain the classes from src/test/java. The generated jar files follow the naming schema artifactId-version.jar for the first one and artifactId-version-classifier.jar for the second one. The parts artifactId, versions will be replaced by the values given within your project pom.xml file. The classifier will be set to tests which is a default of the maven-jar-plugin which can be changed if you need by using the configuration in the jar goal.

Note: The downside of this solution is that you don't get the transitive test-scoped dependencies automatically. Maven only resolves the compile-time dependencies, so you'll have to add all the other required test-scoped dependencies by hand.

The preferred way

In order to let Maven resolve all test-scoped transitive dependencies you should create a separate project.

<project>
   <groupId>groupId</groupId>
    <artifactId>artifactId-tests</artifactId>
    <version>version</version>
  ...
</project>

• Move the sources files from src/test/java you want to share from the original project to the src/main/java of this project. The same type of movement counts for the resources as well of course.
• Move the required test-scoped dependencies and from the original project to this project and remove the scope (i.e. changing it to the compile-scope). And yes, that means that the junit dependency (or any other testing framework dependency) gets the default scope too. You'll probably need to add some project specific dependencies as well to let it all compile again.

Now you have your reusable test-classes and you can refer to it as you're used to:

<project>
  ...
  <dependencies>
    <dependency>
      <groupId>groupId</groupId>
      <artifactId>artifactId-tests</artifactId>
      <version>version</version>
      <scope>test</scope>
    </dependency>
  </dependencies>
  ...
</project>