How to solve “Cause: the class org.apache.tools.ant.taskdefs.optional.junit.JUnitTask was not found.” while running “ant test”?

Question

I have a target named test and I want to do some tests.

I put here the important parts in build.xml. It includes:

Answer 1

In my case (using Mint, based on Debian)

sudo apt-get install ant-optional

was the only thing that worked.