aspectj:test-compile goal should check for pom.build.unitTestSourceDirectory

The aspectj plugin should check for the presence of the unit test
source directory before calling aspectj:compile. This is useful in cases where you want to define your preGoal in your subproject's maven.xml and subprojects don't have any tests defined. Otherwise you'll get a error telling you the 'maven.test.compile.src.set' property doesn't exist. This is easy to add, just add the following check to the aspectj:test-compile goal:

<j:set var="testSourceDir" value="$

"/>
<ant:available
property="hasTestDir"
file="$

"/>

<j:if test="$

">