Overriding configurations from a parent pom can be done by adding the combine.self="override"
attribute to the element in your pom.
Try changing your plugin configuration to:
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-compiler-plugin</artifactId>
<executions>
<execution>
<id>my-testCompile</id>
<phase>test-compile</phase>
<goals>
<goal>testCompile</goal>
</goals>
<configuration combine.self="override">
<fork>true</fork>
<executable>${jdk15.executable}</executable>
<compilerVersion>1.5</compilerVersion>
<source>1.5</source>
<target>1.5</target>
<verbose>true</verbose>
</configuration>
</execution>
</executions>
</plugin>
For more information on overriding plugins, see: http://maven.apache.org/pom.html