How do I include a dependency’s test jar into a Maven project’s deployment?

Better to configure maven pom file in your first module

<project>
    <groupId>com.example</groupId>
    <artifactId>foo</artifactId>
    <version>1.0.0-SNAPSHOT</version>
    <packaging>jar</packaging>
    <build>
        <plugins>
            ...
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-jar-plugin</artifactId>
                <executions>
                    <execution>
                        <goals>
                            <goal>test-jar</goal>
                        </goals>
                    </execution>
                </executions>
            </plugin>

After this a mvn install/release will also deploy an artifact foo-1.0.0-SNAPSHOT-tests.jar

Then configure the dependency on the test jar with classifier (like suggested in other responses)

<dependency>
    <groupId>com.example</groupId>
    <artifactId>foo</artifactId>
    <version>1.0.0-SNAPSHOT</version>
    <type>test-jar</type>
    <!-- uncomment if needed in test scope only
         <scope>test</scope>
    -->
</dependency>