After searching the internet for a few hours and some things I finally found out how.
The goal has to be the report, like this:
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-surefire-report-plugin</artifactId>
<version>2.9</version>
<configuration>
<outputDirectory>${project.build.directory}/site</outputDirectory>
<reportsDirectories>
<reportsDirectories>${project.build.directory}/site/surefire-reports</reportsDirectories>
</reportsDirectories>
</configuration>
<reportSets>
<reportSet>
<id>integration-tests</id>
<reports>
<report>report-only</report>
</reports>
</reportSet>
</reportSets>
</plugin>