BUILD FAILED
path\to\Geoserver\build.xml:259: Could not create task or type of type: junit.
Ant could not find the task or a class this task relies upon.

To fix this problem, you need to copy a JUnit.jar file to your eclipse/plugins/org.apache.ant/lib directory
Next you need to go into Eclipse then click on Window->Preferences->ant->Runtime, then select 'Ant Home Entries (Default). Click on the button 'Add External JARs'. Locate the junit.jar file you copied, select it and hit 'OK'. Hit 'Apply', then 'OK' and try your ant task again. That should fix it.