I finally added this configuration to my POM:
<build>
<testResources>
<testResource>
<directory>${basedir}/src/main/java/</directory>
<includes>
<include>**/*.properties</include>
</includes>
<excludes>
<exclude>**/*.java</exclude>
</excludes>
</testResource>
<testResource>
<directory>${basedir}/src/test/resources/</directory>
</testResource>
</testResources>
</build>
Then I added the properties file with:
.addAsResource("com/example/module/messages.properties")
Now Maven copies my messages.properties
to the directory target/test-classes
. Therefore ShrinkWrap will find it in the classpath.