The maven test phase calls the unit tests through a maven-antrun-plugin, with all necessary jars picked up in the default local repository ${user.home}/.m2/repository :

...
<execution>
  <phase>test</phase>
  <id>runAll</id>
  <configuration>
    <tasks>
      <ant dir="tests" target="runAll">
        <property name="junit.jar" location="${user.home}/.m2/repository/junit/junit/3.8.1/junit-3.8.1.jar" />
        <property name="jakarta.oro.jar" location="${user.home}/.m2/repository/oro/oro/2.0.8/oro-2.0.8.jar" />
...

If the maven repository is not in this standard location, this will make an mvn test fail because these jars cannot be found. It would be better if those jars location was based on a m2_repo property (already available in tests/build.xml) that the user could customize with a 
$ mvn -Dm2_repo=<local maven repository> test :

...
<ant dir="tests" target="runAll">
  <property name="m2_repo" location="${user.home}/.m2/repository"/>
  <property name="junit.jar" location="${m2_repo}/junit/junit/3.8.1/junit-3.8.1.jar" />
  <property name="jakarta.oro.jar" location="${m2_repo}/oro/oro/2.0.8/oro-2.0.8.jar" />